Adapted EBNF as metasyntax notation

[OJacot-Descombes]

We need a syntax notation. Since D# will be line-oriented and (most likely) white-space sensitive I suggest using EBNF, but in a line-oriented and white-space sensitive manner, allowing us to express the D# grammar easily, without the need for end-of-line tokens and indentation tokens.

Example:

IfStatement =
    "if" Expression
        StatementSequence
    { "elsif" Expression
        StatementSequence }
    [ "else" Expression
        StatementSequence ]
;