testing: implement related code (#222252)

* 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

.vscode/extensions/vscode-selfhost-test-provider/tsconfig.json

@@ -12,6 +12,7 @@
 		"../../../src/vscode-dts/vscode.d.ts",
 		"../../../src/vscode-dts/vscode.proposed.testObserver.d.ts",
 		"../../../src/vscode-dts/vscode.proposed.testMessageStackTrace.d.ts",
+		"../../../src/vscode-dts/vscode.proposed.testRelatedCode.d.ts",
 		"../../../src/vscode-dts/vscode.proposed.attributableCoverage.d.ts"
 	]
 }