adopt extended backus-naur form

[tanner0101]

LeafKit should consider officially adopting an EBNF describing its syntax. This could help new users understand the language and help contributors write tooling.

I've taken a first stab at writing the EBNF, but it's not perfect yet.

template     = { text | tag };
tag          = tag start, [ identifier ], [ params ], has body;
has body     = ":";
func         = identifier, params;
number       = { digit };
string       = '"', { text - '"' }, '"';
identifier   = character, { character | digit | "_" };
params       = params start, [ param, { param delim, [ space ], param } ], params end;
param        = identifier | func | string | number;
param delim  = ",";
params end   = ")";
params start = "(";
text         = { ascii | tag escape } - tag start;
tag escape   = "\#";
tag start    = "#";
character    = ? a...z | A...Z ?;
digit        = ? 0...9 ?;
ascii        = ? any byte ?;
space        = " "

A couple of things left to nail down:

• String escaping
• Supported identifier syntax
• White space
• Declarations of basic types like character, digit, ascii, etc