Introduce syntax highlighting for .vue files

Problem to solve

At the moment, the single-file Vue components don't have any syntax highlighting in Web IDE.

Further details

Single file Vue components are common within the GitLab application, so it makes sense to support this for GitLab's own needs as well the broader audience.

Proposal

The outcomes of this issue are two fold:

1. The Web IDE will support proper syntax highlighting of single file Vue components.
2. Documentation will be created for other contributors to help provide support in both GitLab and with upstream contirbutions for other language support.

Further details are in this comment.

Documentation

Documentation should be updated for both supported languages through the main upstream package as well as individual supported languages through contribution to GitLab. This will ensure clear expectations as well as allow users to help contribute additional languages they might need.