New procedural cosmetic operator: `:remove()`

Related issue:
- gorhill/uBlock#2252

The purpose is to outright remove elements from the
DOM tree. Since `:remove()` is an "action" operator,
it must only be used as a trailing operator (just
like the `:style()` operator).

AdGuard's cosmetic filter syntax `{ remove: true; }`
will be converted to uBO's `:remove()` operator
internally.

***

New procedural cosmetic operator: `:upward(...)`

The purpose is to lookup an ancestor element.

When used with an integer argument, it is synonym of
`:nth-ancestor()`, which will be deprecated and which
will no longer be supported once no longer used in
mainstream filter lists.

Filter lists maintainers must only use `:upward(int)`
instead of `:nth-ancestor(int)` once the new operator
become available in all stable releases of uBO.

`:upward()` can also accept a CSS selector as argument,
in which case the nearest ancestor which matches the
CSS selector will be selected.

Co-authored-by:  gorhill <585534+gorhill@users.noreply.github.com>

Related discussion:
- gorhill/uBO-Extra#119 (comment)

As an incidental side effect, this may or may not
prevent execution of the content of some inline
script tags.

Co-authored-by:  gorhill <585534+gorhill@users.noreply.github.com>