* wip
* testing: implement related code
This implements a simple provider-based related code feature, and
adopts it in the selfhost test extension.
It's surfaced to users via automatic delegation in the "run tests at
cursor" command, which I feel like it very natural. I also add two
commands which reuse the `SymbolNavigationAction` to implement
go to/peek views using the references infrastructure. The upstream
change involved in this is the addition of the "go to" preference
for tests.
cc @jrieken @mjbvz
* bump distro for API enablement
This adds an API that extensions can use to contribute 'followup'
actions around test messages. Here just a dummy hello world using
copilot, but extensions could have any action here, such as actions to
update snapshots if a test failed:

Implemented using a simple provider API.
For /fixTestFailures, I want to get more 'real world' tests and test
fixes. This makes a change in the selfhost test provider such that when
a test fails and is then fixed, we record the code changes into a JSON
file in the `.build` directory. In a few days I'll follow up with team
members to collect their test failures and use them as evaluation tests
for copilot. The FailureTracker will be removed when I've gotten enough
data.