Implement the field exists check. This is the '@' token on the
left hand side of '&&' in the expression below.

$.menu.items[?(@ && @.id == 'ViewSVG')].id
ArrayEvalFilter: add support for @.foo.
Delete now-unused HasPathFilter class.
ArrayEvalFilter: add support for Javascript-style regular expressions.
ArrayEvalFilter: use HasPathExpression for '@' too