add luacheck, fix errors, add github actions

.luacheckrc

@@ -0,0 +1,25 @@
+unused_args = false
+allow_defined_top = true
+
+exclude_files = {".luacheckrc"}
+
+globals = {
+    "minetest", "core",
+
+    --mod provided
+
+}
+
+read_globals = {
+    string = {fields = {"split"}},
+    table = {fields = {"copy", "getn"}},
+
+    --luac
+    "math", "table",
+
+    -- Builtin
+    "vector", "ItemStack", "dump", "DIR_DELIM", "VoxelArea", "Settings", "PcgRandom", "VoxelManip", "PseudoRandom",
+
+    --mod produced
+    "default", "screwdriver", "unifieddyes", "computers",
+}