chore: add additional compiler flags (#17815)

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

Resolves: #343

src/nvim/main.c

@@ -2099,6 +2099,7 @@ static bool file_owned(const char *fname)
 /// @param errstr  string containing an error message
 /// @param str     string to append to the primary error message, or NULL
 static void mainerr(const char *errstr, const char *str)
+  FUNC_ATTR_NORETURN
 {
   char *prgname = (char *)path_tail((char_u *)argv0);