[metif12]

What this adds

Test Explorer integration. The extension used tasks and CodeLens for tests, so
nothing appeared in the Test Explorer panel — a user had no tree of tests, no
per-test run button, and no pass/fail status in the panel.

golang.go uses vscode.tests.createTestController with three views in the
test container. This registers a controller and shows one item per test_
function.

Why the mapping is unusually direct

A V test is a test_-prefixed function in a _test.v file. There is no test
framework and no test object, so the file layout is the whole contract. That
makes the three pieces line up:

Test Explorer	V
TestItem	a test_ function
TestRunRequest.include	VTEST_ONLY_FN
run	v test <file> -run-only <name>

So running a selected test is one command with one filter, and the output is
parsed to mark each test passed, failed or errored.

Discovery

Best-effort, so it cannot break activation:

• a workspace with no _test.v files shows an empty tree
• a file that cannot be read is skipped
• the tree refreshes on workspace and file changes, so it stays honest without
  tracking which file changed into which item

Verification

npm run lint, npm run lint:md, npm run test:grammar, npm run build and
npm run package all pass.

npm test reports 29 tests, 26 pass, 3 fail. The three failures are the
pre-existing Windows path-separator tests on master, fixed in
#561 and not touched here.

Proof the new tests catch something. I broke the test-function regex in
testDiscovery.ts and watched finds the test functions in a test file go red,
then reverted it. A test that cannot fail is not evidence.

Known limitations

• Not exercised in a real VS Code window. The controller wiring is not covered by
  the extension-host suite; the discovery and output parsing are.
• A test that is neither passed nor failed in the output is reported as errored
  with "This test did not run", which is a different problem from running and
  failing. That distinction is deliberate.
• v test reports per-file, so a run of a whole file marks every test in it from
  one output. Per-test timing is not available.

Relation to other PRs

Independent of #556, #557, #558, #559, #560, #561 and #562. It adds new files
and touches extension.ts and package.json only where the other PRs do not.