cmd/chrome: go mod tidy

Run go mod tidy on cmd/chroma using go1.22.0

Also add a couple of steps to CI to run as part of CI, and enforce that
the module be tidy by ensuring the git working directory is not dirty at
the end of a workflow.

We use this approach in all of the CUE project workflows:

https://github.com/cue-lang/cue/blob/5b4a5819b55593357486cba3c7fc0d0a1853f031/.github/workflows/trybot.yml#L202-L203

.github/workflows/ci.yml

@@ -13,8 +13,13 @@ jobs:
         uses: actions/checkout@v4
       - name: Init Hermit
         run: ./bin/hermit env -r >> $GITHUB_ENV
+      - name: Ensure cmd/chroma is tidy
+        working-directory: cmd/chroma
+        run: go mod tidy
       - name: Test
         run: go test ./...
+      - name: Check that git is clean at the end of the job
+        run: test -z "$(git status --porcelain)" || (git status; git diff; false)
   lint:
     name: Lint
     runs-on: ubuntu-latest