Add Syntect custom syntax & themes support

[antaalt]

Hello,
I am trying to work with the syntect code editor which is great, the issue I have now is that it seems impossible to load custom syntax & themes to add support for others languages than the one available by default in sublime text, maybe I just missed the API, but it would be nice to be able to load a custom tmTheme and sublime-syntax as Syntect support this via SyntaxSetBuilder.

From what I saw, egui run syntect::parsing::SyntaxSet::load_defaults_newlines() which is created with the cache, but we could somehow allow egui to load custom themes & syntax aswell