Add support for AdGuard's `mp4` filter option

[THEtomaso]

References: https://github.com/uBlockOrigin/uBlock-issues/issues/701/ https://github.com/gorhill/uBlock/commit/68ae847ba385c09c5efa511d18a18a4753af47be/

Note: $empty doesn't show up as invalid in uBO Legacy's logger, so I assume that it is supported already?: https://github.com/gorhill/uBlock/commit/3e5c9e00ab3603ae0c02e08b007b084404bbb71d/

[DandelionSprout]

$mp4 can be treated as an alias for $redirect=noopmp4, from how I've understood it.

[krystian3w]

or $redirect=noopmp4,media and optionaly as duplicated filter with object:

$redirect=noopmp4,object

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uBO legacy need resource/request type for redirects.

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so I assume that it is supported already?:

or omitted. Can you test on any simple anti-adblock pop-up can disabled by $empty?