refactor: fix clang-tidy warnings

Enable and fix bugprone-misplaced-widening-cast warning.

Fix some modernize-macro-to-enum and readability-else-after-return
warnings, but don't enable them. While the warnings can be useful, they
are in general too noisy to enable.

src/nvim/mbyte.c

@@ -1186,9 +1186,8 @@ static int utf_convert(int a, const convertStruct *const table, size_t n_items)
       && a <= table[start].rangeEnd
       && (a - table[start].rangeStart) % table[start].step == 0) {
     return a + table[start].offset;
-  } else {
-    return a;
   }
+  return a;
 }
 
 // Return the folded-case equivalent of "a", which is a UCS-4 character.  Uses