Fix vscode-test setup and add CI job

[rprimet]

The current vscode-test stub was broken by the use of p-queue (promise queue, a dependency when implementing the "run test" feature).

The current p-queue is not compatible with commonjs2 builds, which we use.

This PR pins p-queue to 6.6.2, which is the last version that supports a commonjs build, and adds a CI job that runs vscode-test

Ultimately we should use ESM for packaging our plugin but the support for this in vscode is still not yet completely mature (?) -- see for instance microsoft/vscode#130367 and https://code.visualstudio.com/updates/v1_94#_esm-is-shipping-for-vs-code

[rprimet]

@AltGr if you have comments about the CI setup for opam/dune those are welcome!

Also I added atdgen as a dependency which may not be ideal? (maybe atdgen_runtime should be a runtime dependency and atdgen a build-time only dependency?)

[denismerigoux]

Great!