Replace deprecated String.prototype.substr() (#17949)

* Replace deprecated String.prototype.substr()

.substr() is deprecated so we replace it with .slice() which works similarily but isn't deprecated

* Change String.prototype.substring() to String.prototype.slice()

.substring() and .slice() work very similary but .slice() is a bit faster and stricter

* Add ESLint rule to forbid usage of .substr and .substring

.substr() is deprecated and .substring() is very similar to .slice() so better to use .slice() at all times

Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

app/javascript/mastodon/features/emoji/emoji_mart_search_light.js

@@ -124,7 +124,7 @@ function search(value, { emojisToShowFilter, maxResults, include, exclude, custo
           for (let id in aPool) {
             let emoji = aPool[id],
               { search } = emoji,
-              sub = value.substr(0, length),
+              sub = value.slice(0, length),
               subIndex = search.indexOf(sub);
 
             if (subIndex !== -1) {