[Glimmer] Improve glimmer comments

[GavinJoyce]

With the following input:

<div>
  {{!-- Foo --}}
  {{#if @foo}}
    Foo
  {{/if}}

  {{!-- Bar --}}
  {{#if @bar}}
    Bar
  {{/if}}
</div>

We currently get the following incorrect output:

<div>
  {{! Foo }}
  {{#if @foo}}
    Foo
  {{/if}}{{! Bar }}{{#if @bar}}
    Bar
  {{/if}}
</div>

This PR will change this output to:

<div>
  {{! Foo }}
  {{#if @foo}}
    Foo
  {{/if}}
  {{! Bar }}
  {{#if @bar}}
    Bar
  {{/if}}
</div>

• Rebase once [Glimmer] improve text/mustache formatting #6186 lands
• I’ve added tests to confirm my change works.
• (If changing the API or CLI) I’ve documented the changes I’ve made (in the docs/ directory)
• (If the change is user-facing) I’ve added my changes to the CHANGELOG.unreleased.md file following the template.
• I’ve read the contributing guidelines.

✨Try the playground for this PR✨

[GavinJoyce]

This looks 🍏. I'll rebase and add the change log once #6186 has been merged

[GavinJoyce]

This is ready for review