Add support for Glimmer JS/TS

[IgnaceMaes]

Adds support for .gjs and .gts file formats. Glimmer <template> tag components are the new format which power Ember.js.

References:

• RFC: https://rfcs.emberjs.com/id/0779-first-class-component-templates
• Component implementation: https://github.com/ember-template-imports/ember-template-imports

How do I actually run the tests locally? It would be great it this could be documented in the testing section in the contributing docs.