Add CI checks

.github/workflows/lint.yml

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+name: Linting and style checking
+
+on: [push, pull_request]
+
+jobs:
+  luacheck:
+    name: Luacheck
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v1
+
+      - name: Prepare
+        run: |
+          sudo apt-get update
+          sudo add-apt-repository universe
+          sudo apt install luarocks -y
+          sudo luarocks install luacheck
+
+      - name: Run Luacheck
+        run: luacheck .
+
+  stylua:
+    name: StyLua
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Lint with stylua
+      uses: JohnnyMorganz/stylua-action@1.0.0
+      with:
+        token: ${{ secrets.GITHUB_TOKEN }}
+        args: --check .

.luacheckrc

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+-- Rerun tests only if their modification time changed.
+cache = true
+codes = true
+
+-- Glorious list of warnings: https://luacheck.readthedocs.io/en/stable/warnings.html
+ignore = {
+  "212", -- Unused argument, In the case of callback function, _arg_name is easier to understand than _, so this option is set to off.
+  "411", -- Redefining a local variable.
+  "412", -- Redefining an argument.
+  "422", -- Shadowing an argument
+  "122" -- Indirectly setting a readonly global
+}
+
+-- Global objects defined by the C code
+read_globals = {
+  "vim",
+}

.stylua.toml

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+indent_type = "Spaces"
+indent_width = 2
+no_call_parentheses = true