chore: add repo tooling

.luacheckrc

@@ -0,0 +1,16 @@
+std = "lua52"
+unused_args = false
+max_line_length = 120
+
+read_globals = {
+    "mp",
+    "mpv",
+}
+
+allow_defined_top = true
+
+files["**/*.lua"] = {
+    ignore = {
+        "631", -- allow string.format mismatch warnings common in mpv scripts
+    },
+}