docs: new formatting syntax ideas

docs/getting_started/look_and_feel.md

@@ -100,8 +100,8 @@ performed without sacrificing accuracy. Please see the below example for a quick
       std::cout << std::setw(10) << std::setfill('*') << v2 << '\n';  // ***70 mi/h
       std::cout << std::format("{:*^10}\n", v3);                      // *110 km/h*
       std::println("{:%N in %U of %D}", v4);                          // 70 in mi/h of LT⁻¹
-      std::println("{:{%N:.2f}%?%U}", v5);                            // 30.56 m/s
-      std::println("{:{%N:.2f}%?{%U:dn}}", v6);                       // 31.29 m⋅s⁻¹
+      std::println("{::N[.2f]}", v5);                                 // 30.56 m/s
+      std::println("{::N[.2f]U[dn]}", v6);                            // 31.29 m⋅s⁻¹
       std::println("{:%N}", v7);                                      // 31
     }
     ```
@@ -144,8 +144,8 @@ performed without sacrificing accuracy. Please see the below example for a quick
       std::cout << std::setw(10) << std::setfill('*') << v2 << '\n';  // ***70 mi/h
       std::cout << std::format("{:*^10}\n", v3);                      // *110 km/h*
       std::println("{:%N in %U of %D}", v4);                          // 70 in mi/h of LT⁻¹
-      std::println("{:{%N:.2f}%?%U}", v5);                            // 30.56 m/s
-      std::println("{:{%N:.2f}%?{%U:dn}}", v6);                       // 31.29 m⋅s⁻¹
+      std::println("{::N[.2f]}", v5);                                 // 30.56 m/s
+      std::println("{::N[.2f]U[dn]}", v6);                            // 31.29 m⋅s⁻¹
       std::println("{:%N}", v7);                                      // 31
     }
     ```

docs/index.md

@@ -38,7 +38,7 @@ The library source code is hosted on [GitHub](https://github.com/mpusz/mp-units)
     int main()
     {
       constexpr quantity dist = 364.4 * smoot;
-      std::println("Harvard Bridge length = {:{%N:.5} %U} ({:{%N:.5} %U}, {:{%N:.5} %U}) ± 1 εar",
+      std::println("Harvard Bridge length = {::N[.5]} ({::N[.5]}, {::N[.5]}) ± 1 εar",
                    dist, dist.in(usc::foot), dist.in(si::metre));
     }
     ```
@@ -58,7 +58,7 @@ The library source code is hosted on [GitHub](https://github.com/mpusz/mp-units)
     int main()
     {
       constexpr quantity dist = 364.4 * smoot;
-      std::println("Harvard Bridge length = {:{%N:.5} %U} ({:{%N:.5} %U}, {:{%N:.5} %U}) ± 1 εar",
+      std::println("Harvard Bridge length = {::N[.5]} ({::N[.5]}, {::N[.5]}) ± 1 εar",
                    dist, dist.in(usc::foot), dist.in(si::metre));
     }
     ```

docs/users_guide/framework_basics/faster_than_lightspeed_constants.md

@@ -61,7 +61,7 @@ constexpr auto speed_of_light_in_vacuum = 1 * si::si2019::speed_of_light_in_vacu
 
 QuantityOf<isq::permittivity_of_vacuum> auto q = 1 / (permeability_of_vacuum * pow<2>(speed_of_light_in_vacuum));
 
-std::println("permittivity of vacuum = {} = {:{%N:.3e} %U}", q, q.in(F / m));
+std::println("permittivity of vacuum = {} = {::N[.3e]}", q, q.in(F / m));
 ```
 
 The above first prints the following:

docs/users_guide/framework_basics/simple_and_typed_quantities.md

@@ -75,7 +75,7 @@ Here is a simple example showing how to deal with such quantities:
       const quantity duration = 2 * h;
       const quantity speed = avg_speed(distance, duration);
 
-      std::println("A car driving {} in {} has an average speed of {:{%N:.4} %U} ({:{%N:.4} %U})",
+      std::println("A car driving {} in {} has an average speed of {::N[.4]} ({::N[.4]})",
                    distance, duration, speed, speed.in(km / h));
     }
     ```
@@ -103,7 +103,7 @@ Here is a simple example showing how to deal with such quantities:
       const quantity duration = 2 * h;
       const quantity speed = avg_speed(distance, duration);
 
-      std::println("A car driving {} in {} has an average speed of {:{%N:.4} %U} ({:{%N:.4} %U})",
+      std::println("A car driving {} in {} has an average speed of {::N[.4]} ({::N[.4]})",
                    distance, duration, speed, speed.in(km / h));
     }
     ```
@@ -194,7 +194,7 @@ The previous example can be re-typed using typed quantities in the following way
       const quantity duration = isq::time(2 * h);
       const quantity speed = avg_speed(distance, duration);
 
-      std::println("A car driving {} in {} has an average speed of {:{%N:.4} %U} ({:{%N:.4} %U})",
+      std::println("A car driving {} in {} has an average speed of {::N[.4]} ({::N[.4]})",
                    distance, duration, speed, speed.in(km / h));
     }
     ```
@@ -223,7 +223,7 @@ The previous example can be re-typed using typed quantities in the following way
       const quantity duration = isq::time(2 * h);
       const quantity speed = avg_speed(distance, duration);
 
-      std::println("A car driving {} in {} has an average speed of {:{%N:.4} %U} ({:{%N:.4} %U})",
+      std::println("A car driving {} in {} has an average speed of {::N[.4]} ({::N[.4]})",
                    distance, duration, speed, speed.in(km / h));
     }
     ```

docs/users_guide/framework_basics/the_affine_space.md

@@ -501,7 +501,7 @@ std::println("| {:<18} | {:^18} | {:^18} | {:^18} |",
 std::println("|{0:=^20}|{0:=^20}|{0:=^20}|{0:=^20}|", "");
 
 auto print_temp = [&](std::string_view label, auto v) {
-  std::println("| {:<18} | {:^18} | {:^18} | {:^18{%N:.2f}%?%U} |", label,
+  std::println("| {:<14} | {:^18} | {:^18} | {:^18:N[.2f]} |", label,
                v - room_reference_temp, (v - si::ice_point).in(deg_C), (v - si::absolute_zero).in(deg_C));
 };