chore: add additional compiler flags (#17815)

Added:
  - -Wdouble-promotion
  - -Wmissing-noreturn
  - -Wmissing-format-attribute
  - -Wsuggest-attribute={pure,const,malloc,cold}

Resolves: #343

src/nvim/ex_docmd.c

@@ -6629,6 +6629,7 @@ static void ex_quit(exarg_T *eap)
 
 /// ":cquit".
 static void ex_cquit(exarg_T *eap)
+  FUNC_ATTR_NORETURN
 {
   // this does not always pass on the exit code to the Manx compiler. why?
   getout(eap->addr_count > 0 ? (int)eap->line2 : EXIT_FAILURE);