Ability to declare EXTERNAL functions using C#/JS inside Inky

A super common use case for ink is building a game that relies on some external functions. These can be simple (capitalize the first line of text) and complex (handing game state). Currently this isn't possible within inky, which makes it tricky to test stories without setting up and running the game environment.

This also makes it far more difficult to share working ink source code with others who aren't set up with the entire dev suite.

The pitch is this:
Allow inserting JS/C# either:
- Into your ink project as their own files
- Directly into ink files, wrapped in tags (or something).

Personally I don't mind which language we pick; they both open the same doors.
I'd guess JS is easier, since it's actually running Inky in the first place - same for online tools like Borogrove.
I'm sure others will though, and I suppose the ideal is to have both.