Fix detail::write with fallback formatter (#1829)

* add support for fallback_formatter in detail::write

* add ToString test into OStreamTest

to check fmt::to_string() with class that has output stream operator

* add WithOstreamOperator test into CompileTest

to check fmt::format() with FMT_COMPILE() and class that has output stream operator

* use conditional_t inside detail::write instead of 2 overloads

* Revert "add WithOstreamOperator test into CompileTest"

* remove Context from template parameters in detail::write

test/ostream-test.cc

@@ -319,3 +319,7 @@ TEST(OStreamTest, CopyFmt) {
 TEST(OStreamTest, CompileTimeString) {
   EXPECT_EQ("42", fmt::format(FMT_STRING("{}"), 42));
 }
+
+TEST(OStreamTest, ToString) {
+  EXPECT_EQ("ABC", fmt::to_string(fmt_test::ABC()));
+}