Add support for ABNF

[lidaobing]

Please note: New lexers are contributed to Rouge from the community. Your request will help the community in developing a lexer but it does not mean that anyone will develop the lexer.

ABNF

The syntax of ABNF is defined with ABNF in https://www.rfc-editor.org/rfc/rfc5234#section-4

Implementation in other libraries

• pygements: https://pygments.org/docs/lexers/#pygments.lexers.grammar_notation.AbnfLexer
• chroma: https://github.com/alecthomas/chroma/blob/a56e228d32cf7596525fab122f89d7c1ad112a67/lexers/embedded/abnf.xml#L4

Additional context N/A