Add GitHub Actions CI job that runs vimscript linter

Author: alerque

.github/workflows/vint.yml

@@ -0,0 +1,16 @@
+name: Vint
+on: [push, pull_request]
+jobs:
+  vint:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout
+      uses: actions/checkout@master
+    - name: Set up Python
+      uses: actions/setup-python@v1
+      with:
+        python-version: 3.7
+    - name: Setup dependencies
+      run: pip install vim-vint
+    - name: Run Vimscript Linter
+      run: vint .