Move gulp typestest to a separate GitHub Actions workflow

This way we introduce more parallelism in the GitHub Actions tests, which should reduce overall runtime and thus cannot hurt.
It also means that we'd be able to (temporarily) land PRs with "broken" types, since this workflow won't be `Required`, such that TypeScript won't block general development.

.github/workflows/types.yml

@@ -0,0 +1,34 @@
+name: Types
+on: [push, pull_request]
+permissions:
+  contents: read
+
+jobs:
+  test:
+    name: Test
+    runs-on: ubuntu-latest
+
+    strategy:
+      fail-fast: false
+      matrix:
+        node-version: [lts/*]
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Use Node.js ${{ matrix.node-version }}
+        uses: actions/setup-node@v4
+        with:
+          node-version: ${{ matrix.node-version }}
+
+      - name: Install Gulp
+        run: npm install -g gulp-cli
+
+      - name: Install other dependencies
+        run: npm install
+
+      - name: Run typestest
+        run: gulp typestest

gulpfile.mjs

@@ -2441,7 +2441,6 @@ gulp.task(
   "ci-test",
   gulp.series(
     gulp.parallel("lint", "externaltest", "unittestcli"),
-    "lint-chromium",
-    "typestest"
+    "lint-chromium"
   )
 );