Add toggles to rules

[NyalephTheCat]

When reading the ecma script specs (https://tc39.es/ecma262/multipage/grammar-summary.html#sec-grammar-summary), you'll find rules defined like this:

Basicly it would be good to be able to write rules in a similar way, where it would effectively create multiples rules, like:

IdentifierReference = { Identifier }
IdentifierReference_Yield = { Identifier | yield }
IdentifierReference_Await = { Identifier | await }
IdentifierReference_Yield_Await = { Identifier | yield | await }

You can read about the ecma script notational conventions for this here: https://tc39.es/ecma262/multipage/notational-conventions.html#sec-grammatical-parameters

[tomtau]

So the idea is that a parametrised rule:

IdentifierReference[Yield, Await] = { Identifier ~ [~Yield, "yield"] ~ [~Await, "await"] }

would expand to:

IdentifierReference = { Identifier }
IdentifierReference_Yield = { Identifier | yield }
IdentifierReference_Await = { Identifier | await }
IdentifierReference_Yield_Await = { Identifier | yield | await }

Is that correct? It looks cool. You can perhaps post a thread in #885 for more visibility?

[NyalephTheCat]

For sure!