Update .luacheckrc (#93)

Update .luacheckrc to allow defining global variables, but validate global variables

.luacheckrc

@@ -3,10 +3,11 @@ max_line_length=140
 codes=true
 -- skip showing files with no issues
 quiet=1
--- skip showing undefined variable usage
--- there are thousands of warnings here because luacheck is unaware of Wesnoth's
+-- checks related to undefined variable usage
+-- there can be warnings because luacheck is unaware of Wesnoth's
 -- lua environment and has no way to check which have been loaded
-global=false
+globals={"wesnoth","wml","gui","filesystem","unit_test","stringx","mathx","ai"}
+allow_defined=true
 -- skip showing unused variables
 unused=false
 -- skip showing warnings about shadowing upvalues