Improve the pygments lexer to cover GDScript 2 better

[Qubus0]

we've recently updated the gdscript lexer for our own documentation, this is a port of those changes to fit the godot docs

please check as many pages as you can for errors i might have missed.

also let me know if the colors are fine like this. i think some of the existing colors may be from an older theme and not quite up to date, but i didn't touch them for now.

main changes are functions having proper blue color now, global functions being purple, all classes getting the correct greens (including builtins and types having the different green), and proper colors for $String, string names and node paths

[Qubus0]

Side by side screenshot

[Calinou]

Tested locally, it works as expected. Both dark and light theme seem to work.

This is a great start, as it now looks much closer to the Godot script editor (which is the intended goal).

[AThousandShips]

Any reason this is singular as opposed to other cases?

[Qubus0]

good question. somewhat arbitrary. name, stringescape, comment and a few others are singular too. for the most part plural would make sense, except whitespace seems awkward. perhaps singular everywhere makes the most sense?

[Qubus0]

Name and Punctuation didn't have css classes yet. adding them makes the () and = bluish, pretty hard to notice, and fixes code in warning callouts being yellow