Add native support for custom (graphics context-based) widgets about openrct2-flexui

[Basssiiie]

Thank you for your feature request! What bindings would you need for the custom widget? The custom widget should be possible with widget(), which basically works as a pass-through to OpenRCT2's API, though I was planning to add the custom widget (and any other missing widgets) at some point yes. Maybe naming it graphics() or something similar would be more representative of what it's actually for.

As for exposing the Control class, maybe some day I may expose it. Though, as I've still got a few features in mind I wanna build, I don't want to risk breaking a lot of code dependant on the exposed class. Generally I try to limit breakage of public api's while optimizing internal api's like I see fit. Exposing the control class will prevent me from doing that on this core component, so I think it's better to expose it after all the major features I would want to have are in.

Also yes I should update openrct2.d.ts, I didn't realise it was that outdated. 😅 Though the user should really bring their own openrct2.d.ts and not rely on the one FlexUI uses (is it even in the FlexUI package?).

[ltsSmitty]

My specific issue with the graphics context is that the onDraw requires a this as this first argument, and I wasn't sure how to access it via the widget. Is there a way to do that? Just trying to use this gave me a bunch of complaints and I wasn't sure what to try next, hence me trying to make a new native graphics widget.