Add support for highlighting functions

[Nek-12]

[tmaxxdd]

Hi @Nek-12.

Please provide an example of this feature, perfectly in PR

[Nek-12]

Functions can be highlighted with a different color as on the screenshot based on the parentheses after the declaration. Granted, we can't be sure a function will be highlighted properly with some languages that have function syntax sugar, such as kotlin trailing lambda foo { }. However, we could try to highlight what we can.

[tmaxxdd]

I mark this as "nice to have" as many languages do not have "function keyword" and it would require structure analysis