Add YANG lexer

[gribok]

Based on #1457, I created the lexer in ruby.

Features

• Keywords based on RFC7950
• Seperating keywords in groups, based on BNF definition of RFC7950 for maintenance
• Highlighting of constants, like true, false or deprecated with special token
• Highlighting of namespace-prefix, like type namespace-prefix:counter64;. namespace-prefix will have special token.
  For example namespace urn:ieee:std:802.1AX:yang:ieee802-dot1ax; will have no special token
• Prefix-highlighting should be the complex regex pattern of all pattern. Due to that, I created test cases to validate prefix-highlighting
• Add some test cases for testing of another complex scenarios, like revision-date
• Create example yang file for testing

This will close #1457.

[pyrmont]

Thanks for the submission and I'm sorry it took a while to get to it. I've left some comments in my review. Please let me know if anything is unclear.

One additional point: could you reduce the length of the visual sample? It should be a relatively minimal example of the syntax. As it's currently written, it seems to contain large sections that are duplicative of syntax. You don't need to make sure every line is testing one and only one rule in the lexer but for maintainability reasons, it would be helpful if it contained fewer overlapping sections of code.

[gribok]

Please check my comments.
Appreciate your support. :-)

[pyrmont]

Sorry, I should have said something about this earlier but I removed a lot of the duplication from the visual sample. Let me know if you think it all looks good to you.

[gribok]

Sorry, I should have said something about this earlier but I removed a lot of the duplication from the visual sample. Let me know if you think it all looks good to you.

Thanks! I have adjust the visual sample to an simpler example and additionally removed the long descriptions.

From my point of view, everything looks good now.

[pyrmont]

@gribok Thanks for all your work to get this done—it's finally finished! This will be part of the next release of Rouge. That's scheduled to come out on Tuesday 14 April 🎉