Document highlights provider

[DylanPiercey]

Description

Implements document highlight provider (currently for stylesheets).

Checklist:

• [x] I have updated/added documentation affected by my changes.
• [x] I have added tests to cover my changes.

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 44b91a8ae05cac8f411d6aefba85694e1d1e5f21

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

| Name | Type | | ---------------------- | ----- | | marko-vscode | Patch | | @marko/language-server | Patch |

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