Configure TDD for custom filesystem module for client-side VS Code

Description

So that we're not blocked by upstream changes (see #355019 (closed)), let's set up some TDD test cases for the custom filesystem module that we're building out.

We initially went with a git-like implementation, and some of this work was already started https://gitlab.com/gitlab-org/frontend/playground/gitlab-vscode/-/commit/0711c8492096918ee4f3f03c3be80f0fb59c403c

However, the direction now has changed to an implementation based on OverlayFS, which has been started herre: Replacing current fs with BrowserFS.OverlayFS (gitlab-web-ide!18 - merged)

Tasks

• Reorganize gitlab-vscode project into "packages" with yarn workspaces (this is a related quality-of-life improvement)
• Formulate test scenarios
• Create skipped failing tests