Add flags support for ':matches-css()' regex patterns

@realodix commented on [Thu Nov 25 2021](https://github.com/AdguardTeam/FiltersRegistry/issues/545)

Looks like AdGuard doesn't support regex flags for `:matches-css()`. I didn't notice until I saw https://github.com/AdguardTeam/AdguardFilters/issues/101083 and read the [documentation](https://kb.adguard.com/en/general/how-to-create-your-own-ad-filters#pseudo-class-matches-css), because in the diff.txt file there is no message.

```html

#to-be-blocked::before {
content: "Advertisement"
}

`##div.banner:matches-css-before(content: /adv/i)`

uBO requires using the `i` flag to be case insensitive.

https://github.com/gorhill/uBlock/wiki/Procedural-cosmetic-filters#subjectmatches-cssarg