vscode-bnf for YAML's BNF flavor

This extension adds BNF syntax highlighting to Visual Studio Code.

Files with the extension .bnf will automatically be highlighting.

Some differences with standard BNF grammar:

• /* C-style comments */
• Unquoted unicode escape sequences like #xFEFF
• Production names can have a parenthesized context like s-indent(<n)
• Add operators ⇒ and ×