Add pre-launch task

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Unfortunately, something like this doesn't work in settings.json:

"extension-test-runner.debugOptions": {
  "preLaunchTask": "Watch src & test"
},

since the debugOptions "only" support these Node.js-specific configuration attributes, which don't include the key preLaunchTask.

Is there another way to achieve this that I don't know of? This would be great since otherwise we have to remember to spawn the watch task before running tests to not get outputs based on outdated, transpiled code.

Our .vscode-test.mjs:

import { defineConfig } from "@vscode/test-cli";

export default defineConfig({
  files: "out/test/**/*.test.js",
  version: "stable",
  workspaceFolder: "tests/fixtures",
});