Support `onShift` hook

[DmitrySoshnikov]

Currently we have a semantic action per the whole production, which is called onReduce action. For a better parser-lexer communication we could build a semantic action per each symbol on RHS ("before", and "after" hooks). Alternatively we can provide a generic onShift hook which can receive a production, symbol, lexer/parser instance, etc, and do some action (e.g. set a lexer state).

function onBeforeShift(grammarSymbol, production, lexer, parser, ...) {
  // do actions on shift
}

function onShift(...) { ... }


Object : "{"  Props "}" { $$ = $1 + $2 }

Here onBeforeShift, and onShift is called for { and }.

[DmitrySoshnikov]

Fixed in 8b63df8.