Support VS Code testing API

[matklad]

https://code.visualstudio.com/api/extension-guides/testing

We might want to support that.

The way I understand it, what VS Code wants is a global view of a hierarchy of all the tests in a project. There are two potential sources for such view:

• statically discovered tests (eg, re-run cargo test -- --list on every change, or use symbol index to list every #[test] function)
• external sources -- display a result of a test run

I don't think we can support the first case correctly: in the limit, it is a global view of all the tests, and to maintain that we'd have to do O(N) work on every change worst-case. Displaying a hierarchy of tests also feels like it duplicates other navigation functionality.

So I think we should do the following:

• on the server side, provide the API to fetch project model, such that the vscode can draw one node per cargo package/target
• on the client side, change our test runners to optionally pass in the --message-format json flag, and to populate nodes bellow cargo target with test results.

[matklad]

better to track this in #3601