FIX: emoji autocomplete triggering incorrectly

Previous to this fix the we were checking for non letters.

This was mismatching what pretty-text/addon/emoji.js was doing.

`ù:su`

and

`1:su`

Would lead to an emoji autocomplete popup in the composer.

app/assets/javascripts/discourse/app/components/d-editor.js

@@ -436,7 +436,7 @@ export default Component.extend({
           return false;
         }
 
-        const matches = /(?:^|[^a-z])(:(?!:).?[\w-]*:?(?!:)(?:t\d?)?:?) ?$/gi.exec(
+        const matches = /(?:^|[>.,\/#!$%^&*;:{}=\-_`~()])(:(?!:).?[\w-]*:?(?!:)(?:t\d?)?:?) ?$/gi.exec(
           text.substring(0, cp)
         );