Proposal: Support custom text transformation plugins about maplibre-gl-js

[HarelM]

We have recently made a huge refactoring to the rtl text plugin to split worker code and main thread code.
I think an initial POC would be to try and present a different plugin that works similarly to how rtl text plugin works.
Once this is in place we can decide how to adapt the API to support multiple plugins.
The code in the worker can be changed to support more than the shape arabic method to avoid the need keep this "legacy" name.
Generally speaking, this sounds like a good hook.
A good example of how to write such a plugin is critical for this to work. We could add it to the developer guides.
Cc: @zhangyiatmicrosoft