[Project] Review UI testing process

[mattmasson]

Our current UI testing is done using vscode-extension-tester.

The official guidance from the VSCode team is to use @vscode/test-electron and the new @vscode/test-cli packages.

We hit some issues with older versions of the VSCode teams UI framework, but it would be worth investigating if things are working better now.

[mattmasson]

I hit a number of challenges with this extension when updating dependencies (#294).

1. Compatibility issues with latest NodeJS 18 LTS
2. Stability issues on latest releases (npmjs counts show most people are still on an older version)
3. Frequent failures - our pipelines require retry logic
4. No way to limit which extensions are loaded (may be resolved in later versions)

I have actually been unable to get the E2E tests to run successfully on my local dev machine (but they do pass, after multiple retries, from the PR pipeline).