fix: add luacheck configuration

.luacheckrc

@@ -0,0 +1,18 @@
+-- Global objects
+globals = {
+  "astronvim",
+  "astronvim_installation",
+  "vim",
+  "bit",
+}
+
+-- Rerun tests only if their modification time changed
+cache = true
+
+-- Don't report unused self arguments of methods
+self = false
+
+ignore = {
+  "631", -- max_line_length
+  "212/_.*", -- unused argument, for vars with "_" prefix
+}