[bnf] add bnf for deeplang

[mutouyuguo]

write the bnf for deeplang, just to be familiar with the language, and prepare for lexer and parser.

[mutouyuguo]

• prepare use https://github.com/BNFC/bnfc
• https://bnfc.readthedocs.io/en/latest/

[Mepy]

deeplang-lbnf.zip