Custom keybinds

[TeraByte-Official]

Check all that apply, then describe the issue below:

• [x] I have searched the tracker for similar issues (including closed ones)
• [x] I have provided a detailed example for how this feature should work
• [x] I have provided a use case for this feature, e.g. short description/share link of a game you're making and how you'd use the feature in that game, other things you've tried, edge cases, etc. (if applicable)

The custom_keybind keyword can be put in the header to introduce a custom keybind. For example, if you put custom_keybind h foo in the header, you can put foo before a rule to make it apply when you press H. This can be useful for a recreation of Tres Undos.

[Auroriax]

Hmm, I remember looking into something like this previously but the engine is pretty hard-coded around the existing five buttons...

Similar to https://github.com/Auroriax/PuzzleScriptPlus/issues/97