refactor: use pratt parser for if expressions - WIP, DON'T MERGE

So, here is the (very) first version of the Pratt's parser being used for expressions contained inside {if} statements. This still needs some polishing and there are 4 skipped tests but opening this PR now for:
* code review
* tests checks in all browsers on Sauce
we should may improve the comment to say that this rule will simply catch the expression as text and that the actual parsing will be done in the the JS post-processing through the Pratt parser