eng: onboard to the extension test runner (#195570)

* eng: onboard to the extension test runner

Adds a `.vscode-test.js` file that uses the new extension test CLI to
run tests. Also, onboards the markdown-language-features as the first
built-in extension to use it.

With the `ms-vscode.extension-test-runner` extension installed, the
markdown-language-features' tests can be run and debugged easily in
the UI :)

* fixup

.vscode/settings.json

@@ -6,6 +6,7 @@
 		".build": true,
 		".profile-oss": true,
 		"**/.DS_Store": true,
+		".vscode-test": true,
 		"cli/target": true,
 		"build/**/*.js": {
 			"when": "$(basename).ts"
@@ -143,6 +144,11 @@
 			"${workspaceFolder}/build/**/*.js"
 		]
 	},
+	"extension-test-runner.debugOptions": {
+		"outFiles": [
+			"${workspaceFolder}/extensions/*/out/**/*.js",
+		]
+	},
 	"githubPullRequests.assignCreated": "${user}",
 	"githubPullRequests.defaultMergeMethod": "squash",
 	"githubPullRequests.ignoredPullRequestBranches": [