Add Glimmer JS language

[gilest]

Description

Adds support for Glimmer.js which will be the component authoring format of the next Edition of Ember.js.

Re-stage of #6578 which was reverted due to a scope conflict. The scope conflict has been resolved upstream in lifeart/vsc-ember-syntax/pull/48

Checklist:

• I am adding a new language.
  • The extension of the new language is used in hundreds of repositories on GitHub.com.
    • Search results for each extension:
      • .gjs 2.2k files excluding forks
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • class.gjs
      • template-only.gjs
    • Sample license(s):
      • class.gjs - (https://github.com/discourse/discourse-ai) MIT
      • template-only.gjs (https://github.com/CrowdStrike/opensource.crowdstrike.com) MIT
  • I have included a syntax highlighting grammar:
    • https://github.com/lifeart/vsc-ember-syntax – MIT
  • I have added a color
    • Hex value: #F5835F
    • Rationale: Logo colour from https://glimmerjs.com/ at time of writing