Add to builtin list: "cd"

[Liquidream]

Looks like cd is missing from the list in lua.py
...which can cause issue when minifier is allowed to use it as an available identifier.
(Can't see any others missing tho! 😉)

NOTE: I did try adding cd to the exclusions.txt file, but it still seemed to allow it to be used? 🤔

[Liquidream]

Just ran into this one again while updating SCUMM-8.
(Workaround for now is I redefine cd on start-up, but it's using extra tokens)

I see others have suggested PR's with more missing built-ins, so this issue is less relevant I guess. 🤷‍♂️