Add GDScript language support

[leonidboykov]

Added support for Godot GDScript language. Mostly copied from Pygments and Chroma versions.

Resolves #1016 and gitlab-org/gitlab-ce#19663.

I've also pushed PR #1034 because Str::Affix is missing in rouge, but it is used in original lexers (I've used Str::Char as replacement).
Update: replaced Str::Char with Str::Affix.

[pyrmont]

Sorry it's taken so long to get to this. Please feel free to ask if you have any questions!

[leonidboykov]

@pyrmont I've put variables behind methods, but I am not sure about other issues. These rules are based on Pygments implementation, I'm neither Ruby developer nor regex guru :)

[pyrmont]

Some more comments.

[leonidboykov]

@pyrmont sorry for a long delay. I've tried to fix all the issues with the rules.

[pyrmont]

@leonidboykov No worries. Thanks for all the work! There are 2 outstanding points:

1. This comment about the use of backslashes to continue code across multiple lines.
2. This comment regarding an incorrect use of groups.

[leonidboykov]

@pyrmont I've committed fixes for that.

[pyrmont]

@leonidboykov Thanks for going through all the back and forth with me and I'm sorry for all the mistakes during the code review. I made quite a bit of extra work for you :(

On the positive side, it's great to have this lexer in Rouge! 🎉 We're currently on a two-week release cadence and the next release (3.7.0) will go out next Tuesday so this will very shortly be available as part of the gem. Thanks for making Rouge better for all its users :)