implementation_meta_issue template missing codemirror and highlight.js

There are a handful of Dart language parsers that should be updated with any change to the syntax:

* [highlight.js](https://highlightjs.org/), which is how api.dart.dev and api.pub.dev highlight syntax.
* [codemirror](https://codemirror.net/), which is how DartPad and Cider highlight syntax.
* Grok/Kythe also has their own, but I think that is covered by the existing "Grok support" bullet.

I think the top two deserve their own bullet points in https://github.com/dart-lang/language/blob/master/templates/implementation_meta_issue.txt.

WDYT?

Open templates/implementation_meta_issue.txt and review the existing implementation bullets, especially the Grok support entry. Add separate bullets for highlight.js and CodeMirror, then verify that the template clearly identifies both syntax-highlighting implementations alongside the existing entries.

Written by the indexing model from the issue text.