Add YANG lexer

[gribok]

Based on #347, I created the lexer in golang.

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

[alecthomas]

Hi, thanks! CI failed but it looks minor.

[gribok]

Hi, thanks! CI failed but it looks minor.

Fixed it. Used the fix, to alphabetise the keywords lists as well.