ci: cache fixtures

Only generate fixtures if any grammar from any parser or the parser
generation itself has changed.

.github/actions/cache/action.yml

@@ -0,0 +1,25 @@
+name: 'Cache'
+description: "This action caches fixtures"
+outputs:
+  cache-hit:
+    description: 'Cache hit'
+    value: ${{ steps.cache_output.outputs.cache-hit }}
+runs:
+  using: "composite"
+  steps:
+    - uses: actions/cache@v4
+      id: cache_fixtures
+      with:
+        path: |
+          test/fixtures/grammars
+          target/release/tree-sitter-*.wasm
+        key: fixtures-${{ join(matrix.*, '_') }}-${{ hashFiles(
+          '.github/workflows/builds.yml',
+          '.github/workflows/sanitize.yml',
+          'cli/src/generate/**',
+          'script/generate-fixtures*',
+          'test/fixtures/grammars/*/**/src/*.c') }}
+
+    - run: echo "cache-hit=${{ steps.cache_fixtures.outputs.cache-hit }}" >> $GITHUB_OUTPUT
+      shell: bash
+      id: cache_output