XFA - fix expression parsing & evalution in SOM expressions

[calixteman]

See:

pdf.js/src/core/xfa/som.js

Lines 106 to 116 in 45329af

	case "[":
	// TODO: FormCalc expression so need to use the parser
	operator = operators.dotBracket;
	break;
	case "(":
	// TODO:
	// Javascript expression: should be a boolean operation with a path
	// so maybe we can have our own parser for that stuff or
	// maybe use the formcalc one.
	operator = operators.dotParen;
	break;