Add support for YARA 4.2.0

[TadeasKucera]

There are some new things in YARA 4.1.0 which we'll need to implement:

• New operators icontains, endswith, iendswith, startswith, istartswith.
• Accept \t escape sequence in text strings.
• Consider the behaviour of undefined expressions (YARA goes back to propagating undefined arguments of expressions so that the expression is then undefined).

Please see more and consider all the changes on yara upstream