Add neovim nightly check workflow

.github/workflows/neovim-nightly-test.yml

@@ -0,0 +1,29 @@
+name: Neovim Nightly compatibility test
+on:
+  schedule:
+    - cron: '0 12 * * *'
+
+  workflow_dispatch:
+
+
+jobs:
+  bootstrap-and-check-nightly:
+    strategy:
+      matrix:
+        platform: [ubuntu-latest, macos-latest, windows-latest]
+    runs-on: ${{ matrix.platform }}
+    steps:
+      - uses: actions/checkout@v2
+      - name: Run bootstrap
+        run: make bootstrap
+        env:
+          DOTFILES_CI: 1
+      - name: Install neovim
+        uses: rhysd/action-setup-vim@v1
+        with:
+          neovim: true
+          version: nightly
+      - name: Run neovim checks
+        run: make check_neovim
+        env:
+          DOTFILES_CI: 1