feat(workflow): theming support for the editor area webviews

Description

The MR introduces the theming support for the LS webviews, which are opened in the editor area instead of just those in the panels.

In addition, the MR introduces the theming adapter to drive the "translations" of VSCode theme CSS variables into the unified variables expected by the webviews.

This is part of the bigger effort of Support IDE themes for workflow (gitlab-org/editor-extensions/gitlab-lsp#381) with the LS counter-part being in review as well.

There are no visual changes in this MR (those will become apparent when feat(workflow): pick up theming from VSCode (gitlab-org/editor-extensions/gitlab-lsp!962 - merged) is merged), but to see how this MR and the the LS counter-part work together, I have recorded a quick video:

Related Issues

References gitlab-org/editor-extensions/gitlab-lsp#381

How has this been tested?

Screenshots (if appropriate)

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes