Add support for network filter option `top=`

This allows to match against the hostname of the top-most context,
which usually is the URL in the address bar of the browser.

Related issue/discussion:
- https://github.com/uBlockOrigin/uBlock-issues/issues/3151
- https://github.com/uBlockOrigin/uAssets/issues/17807#issuecomment-1806971916

The `top=` option is compatible with MV3 DNR's `topDomains` and
`excludedTopDomains` as long as the values are plain hostnames
(entity- and regex-like values are discarded).