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/lua/converter.c

@@ -906,9 +906,8 @@ Float nlua_pop_Float(lua_State *lstate, Error *err)
   lua_pop(lstate, 1);
   if (table_props.type != kObjectTypeFloat) {
     return 0;
-  } else {
-    return (Float)table_props.val;
   }
+  return (Float)table_props.val;
 }
 
 /// Convert lua table to array without determining whether it is array

src/nvim/lua/stdlib.c

@@ -378,15 +378,14 @@ int nlua_setvar(lua_State *lstate)
     if (di == NULL) {
       // Doesn't exist, nothing to do
       return 0;
-    } else {
-      // Notify watchers
-      if (watched) {
-        tv_dict_watcher_notify(dict, key.data, NULL, &di->di_tv);
-      }
-
-      // Delete the entry
-      tv_dict_item_remove(dict, di);
     }
+    // Notify watchers
+    if (watched) {
+      tv_dict_watcher_notify(dict, key.data, NULL, &di->di_tv);
+    }
+
+    // Delete the entry
+    tv_dict_item_remove(dict, di);
   } else {
     // Update the key
     typval_T tv;