Don't highlight mention when it's prepended by a dot

src/components/compose.jsx

@@ -108,7 +108,7 @@ function countableText(inputText) {
 // https://github.com/mastodon/mastodon/blob/c03bd2a238741a012aa4b98dc4902d6cf948ab63/app/models/account.rb#L69
 const USERNAME_RE = /[a-z0-9_]+([a-z0-9_.-]+[a-z0-9_]+)?/i;
 const MENTION_RE = new RegExp(
-  `(^|[^=\\/\\w])(@${USERNAME_RE.source}(?:@[\\p{L}\\w.-]+[\\w]+)?)`,
+  `(^|[^=\\/\\w.])(@${USERNAME_RE.source}(?:@[\\p{L}\\w.-]+[\\w]+)?)`,
   'uig',
 );