Enable the ESLint prefer-const rule globally (PR 11450 follow-up)

Please find additional details about the ESLint rule at https://eslint.org/docs/rules/prefer-const

With the recent introduction of Prettier this sort of mass enabling of ESLint rules becomes a lot easier, since the code will be automatically reformatted as necessary to account for e.g. changed line lengths.

Note that this patch is generated automatically, by using the ESLint `--fix` argument, and will thus require some additional clean-up (which is done separately).

src/shared/util.js

@@ -804,7 +804,7 @@ function utf8StringToString(str) {
 }
 
 function isEmptyObj(obj) {
-  for (let key in obj) {
+  for (const key in obj) {
     return false;
   }
   return true;