ci: run parser tests and stylua on push

Two jobs on every push and pull request:

- `test`: installs Neovim stable and plenary.nvim, then runs the
  parser spec suite headlessly via `PlenaryBustedDirectory`.
- `stylua`: checks formatting of `lua/` and `tests/` against the
  repo's `stylua.toml`.

Author: rashedInt32

.github/workflows/test.yml

@@ -0,0 +1,41 @@
+name: Tests
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  test:
+    name: Parser tests
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install Neovim
+        uses: rhysd/action-setup-vim@v1
+        with:
+          neovim: true
+          version: stable
+
+      - name: Install plenary.nvim
+        run: |
+          git clone --depth 1 https://github.com/nvim-lua/plenary.nvim \
+            "$HOME/.local/share/nvim/site/pack/vendor/start/plenary.nvim"
+
+      - name: Run tests
+        run: |
+          nvim --headless --noplugin \
+            -u tests/minimal_init.lua \
+            -c "PlenaryBustedDirectory tests/ {minimal_init='tests/minimal_init.lua',sequential=true}"
+
+  stylua:
+    name: StyLua formatting
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: JohnnyMorganz/stylua-action@v4
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          version: latest
+          args: --check lua/ tests/