Add LuaCheck and fix warnings

.github/workflows/build.yml

@@ -0,0 +1,11 @@
+on: [push, pull_request]
+name: build
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: lint
+        uses: Roang-zero1/factorio-mod-luacheck@master
+        with:
+          luacheckrc_url: ""

.luacheckrc

@@ -0,0 +1,13 @@
+unused_args = false
+max_line_length = false
+
+globals = {
+    "coloredwood"
+}
+
+read_globals = {
+    table = {fields = {"copy", "getn", "contains"}},
+
+    "minetest", "default",
+    "unifieddyes", "stairsplus",
+}

init.lua

@@ -28,7 +28,6 @@ local function is_stairsplus(name, colorized)
 
 	local class = string.sub(name, a+1, b-1) -- from colon to underscore is the class
 	local shape = ""
-	local rest
 	local colorshape
 
 	if class == "stair"