feat(foam-vscode): add jest for testing

[jsjoeio]

This adds all the necessary pieces for Jest to work properly in foam-vscode. It also enables testing to run as part of the GitHub workflow.

[jevakallio]

@jsjoeio thank you for starting this work! I've just pushed a failing commit onto this branch to demonstrate an issue we need to address.

The problems is that the vscode dependency is injected by the VS Code extension host environment (not sure how they provide it; possibly a custom node.require implementation, or a runtime-injected module). This means that any module that imports vscode can not be imported into a jest test without mocking the vscode module first.

Here's an issue on vscode-test with some workarounds. I'm not sure which is the best approach, haven't had time to investigate in depth. Happy to entertain any option.
microsoft/vscode-test#37

Beyond simple unit tests, here's resources on how we could run VSCode e2e tests with Jest:
https://medium.com/@soloydenko/end-to-end-testing-vs-code-extensions-via-jest-828e5edfeb75

The former needs to be addressed before it makes sense to land this. The latter can be left out of scope for further enhancement.

[jsjoeio]

@jevakallio Those are good points! The intention behind this was to have the bare minimal of Jest set up so that we could at least write unit tests.

This means that any module that imports vscode can not be imported into a jest test without mocking the vscode module first.

Right! Thanks for bringing that up. My thinking was to add support for those types of integration and e2e tests later. But if you want that to all land as one unit in this PR, happy to do some investigation (thanks for the resources btw!).

[jsjoeio]

After doing a quick investigation, it seems like mocking the vscode module is the most common approach. There is an ongoing discussion about making this easier for folks.

• Red Hat's [[vs-code]] extension tester
• example [[vs-code]] mock
• end-to-end [[testing]] with [[vs-code]] link
• open issue
• mock with jest
• another link with some resources

For now, I'm going to work on adding a mock then I'll re-request your review.

[jsjoeio]

This was enough to get the test you pushed to pass. Thoughts?

[riccardoferretti]

I think this is a really good step forward!
Even if we can't access vscode, we can still limit its code to the extension.ts file as suggested in one of the links you included (or at least limit its creep into the BL code).

Looks good to me!

[jevakallio]

LGTM! 👍 Thanks @jsjoeio!