patternProperties: check regex engine matching more completely

* Regex matches are not anchored.
* Matching is case insensitive.
bignum.json: add a real number test

The only test was to recognize a number as an integer. Add a test to check a
mathematical operation on such a numer. And take a float.
Add corner case tests for ECMA 262 regexes

Add a very bizarre construct which only ECMA 262 allows in regexes. Check that
lookbehinds are _not_ supported.