Update yarn dev-vscode to build tests with dev mode (i.e. watch and recompile)

[seeM]

Previously, if you ran the VS Code extension's tests from the test pane it would not trigger a recompile and would run stale tests. You had to use yarn test-vscode.

This PR updates yarn dev-vscode (more specifically, apps/vscode/build.ts) to also build tests with dev mode (i.e. a watch server that automatically recompiles).

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[vezwork]

@seeM I am trying to understand the workflow you are describing. What is the test pane? Is this it? I don't see any tests here when I open the repo.

My personal workflow is to keep yarn dev-vscode running in the background while I'm working in the repo, I think this continuously recompiles the tests right? But if I understand correctly, it does not feel good to be able to run stale tests (regardless of workflow) with the yarn test-vscode command. This PR makes it so running yarn test-vscode first recompiles right? Does it also keep the build water running in the background after?

[vezwork]

@seeM explained to me in Slack:

Today:

• yarn dev-vscode does not compile tests
• yarn test-vscode does recompile (on every run) and run tests

Running a test from the extension above does not recompile tests

After PR:

• yarn dev-vscode additionally watches & compiles tests
• yarn test-vscode as before

Running a test from the extension above still does not recompile, but dev-vscode will now handle that for us

He is using this extension to populate the test pane with extension tests: https://marketplace.visualstudio.com/items?itemName=ms-vscode.extension-test-runner

[vezwork]

I tried this PR locally and was still getting stale tests when using the extension. I noticed the test-out dir was not getting cleared, and after manually deleting it and rebuilding, the stale tests went away. I added a commit that adds a small custom plugin to the build that clears the outdir (test-out in the case of the test build) before putting the built files back into it.

[vezwork]

Working well for me locally! A bit hard to test, but it should address all stale test issues we've been facing and doesn't have much of a downside.