Add type tests to GitHub Actions config

- This tests against the same minimum version as the existing type
  definitions on DefinitelyTyped, so users can upgrade to at least the
  version supported there, and then switch over to using this. Note
  that type tests do *not* use Ember Try, and therefore do not drift
  the lockfile. This is intentional: the emitted types here are wholly
  unaffected by the rest of the dependencies other than those in
  `@types`, and for compatibility with TS we only care about the types.
  This makes that part of CI run *much* faster.

- The type tests allow failures against the `typescript@next` version,
  just as the runtime tests do for Ember Canary releases.

.github/workflows/CI.yml

@@ -76,3 +76,26 @@ jobs:
         run: yarn install --frozen-lockfile
       - name: Run Tests
         run: node_modules/.bin/ember try:one ${{ matrix.scenario }} --skip-cleanup
+
+  types:
+    name: type tests
+
+    runs-on: ubuntu-latest
+
+    strategy:
+      fail-fast: false
+      matrix:
+        ts-version:
+          - 4.4
+          - 4.5
+          - 4.6
+          - next
+
+    steps:
+      - uses: actions/checkout@v3
+      - run: yarn install --frozen-lockfile --non-interactive
+      - run: yarn add -D typescript@${{ matrix.ts-version }}
+      - run: ./node_modules/.bin/tsc --project type-tests
+      - run: ./node_modules/.bin/tsc --project type-tests/with-registry
+
+    continue-on-error: ${{ matrix.ts-version == 'next' }}