refactor: knot unit helper renamed to kn in FPS

Relates to #357

example/literals/foot_pound_second.cpp

@@ -108,7 +108,7 @@ int main()
   auto iowa = Ship{.length{860._q_ft},
                    .draft{37._q_ft + 2._q_in},
                    .beam{108._q_ft + 2._q_in},
-                   .speed{33_q_knot},
+                   .speed{33_q_kn},
                    .mass{57'540_q_lton},
                    .mainGuns{16_q_in},
                    .shellMass{2700_q_lb},
@@ -119,7 +119,7 @@ int main()
   auto kgv = Ship{.length{745.1_q_ft},
                   .draft{33._q_ft + 7.5_q_in},
                   .beam{103.2_q_ft + 2.5_q_in},
-                  .speed{28.3_q_knot},
+                  .speed{28.3_q_kn},
                   .mass{42'245_q_lton},
                   .mainGuns{14_q_in},
                   .shellMass{1'590_q_lb},