Support CSS selectors mixed w/ operators in procedural cosmetic filters

Related issue:
- #3683

This commit further increases uBO's procedural cosmetic filters
Adguard's cosmetic filter syntax -- specifically those procedural
cosmetic filters where plain CSS selectors appeared following
a procedural oeprator (this was rejected as invalid by uBO).

Also, experimental support for `:watch-attrs` procedural
operator, as discussed in <uBlockOrigin/uBlock-issues#341 (comment)>.
Support may be dropped before next release depending on whether
a better solution is suggested.

Additionally, the usual opportunistic refactoring toward ES6
syntax.