build: Add action for running luacheck

Run luacheck on CI to catch subtle errors.

Add a luacheckrc that ensures current code issues no warnings. We should
eventually try to remove many of these ignores.

.github/workflows/luacheck.yml

@@ -0,0 +1,26 @@
+name: Linting
+
+on: [push, pull_request]
+
+jobs:
+  luacheck:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+      with:
+        # Subdirectory to avoid linting luarocks code. Use same dir as testy.
+        path: batteries
+    - name: Setup Lua
+      uses: leafo/gh-actions-lua@v8
+      with:
+        luaVersion: 5.4
+    - name: Setup Lua Rocks
+      uses: leafo/gh-actions-luarocks@v4
+    - name: Setup luacheck
+      run: luarocks install luacheck
+    - name: Run Code Linter
+      run: |
+        cd batteries
+        luacheck .

.luacheckrc

@@ -0,0 +1,46 @@
+return {
+    std = "lua51+love",
+    ignore = {
+        "143", -- Accessing an undefined field of a global variable.
+        "211", -- Unused local variable.
+        "212", -- Unused argument.
+        "212/self", -- Unused argument self.
+        "213", -- Unused loop variable.
+        "231", -- Local variable is set but never accessed.
+        "311", -- Value assigned to a local variable is unused.
+        "412", -- Redefining an argument.
+        "413", -- Redefining a loop variable.
+        "421", -- Shadowing a local variable.
+        "422", -- Shadowing an argument.
+        "423", -- Shadowing a loop variable.
+        "423", -- Shadowing a loop variable.
+        "432", -- Shadowing an upvalue argument.
+        "611", -- A line consists of nothing but whitespace.
+        "612", -- A line contains trailing whitespace.
+        "614", -- Trailing whitespace in a comment.
+        "631", -- Line is too long.
+    },
+    files = {
+        ["tests.lua"] = {
+            ignore = {
+                "211", -- Unused local variable. (testy will find these local functions)
+            },
+        },
+        ["assert.lua"] = {
+            ignore = {
+                "121", -- Setting a read-only global variable. (we clobber assert)
+            },
+        },
+        ["init.lua"] = {
+            ignore = {
+                "111", -- Setting an undefined global variable. (batteries and ripairs)
+                "121", -- Setting a read-only global variable. (we clobber assert)
+            },
+        },
+        ["sort.lua"] = {
+            ignore = {
+                "142", -- Setting an undefined field of a global variable. (inside export)
+            },
+        },
+    }
+}