Offer MAME core as a download, like FFmpeg

[YoshiRulz]

It's the largest contributor to the download size by far at over 25% (edit: with e0e5a68, it's even more).

Having a separate update mechanism to EmuHawk would also allow for multiple options, for example the choice between "all arcades" (as 2.9) and "everything". (It would also allow for a separate release cadence, but I'm not sure how useful patching just the Waterbox guest side would be.)
On Discord, there's been some discussion over what we should do now that MAME has dropped the MAME/MESS distinction (now tracking as #4289). If we came up with a categorisation that split the machine list into 4 subsets, then we could offer (at least) 5 builds—1 for each, plus 1 with everything.

[Morilli]

Add citra to that list.

[vadosnaprimer]

Offering the whole MESS part is impossible because of the crazy infrastructure that we'd have to implement, and most MESS cores are worse than currently accepted on tasvideos. Particular MESS cores could work, like game and watch, or other things we don't have better options for. But they don't get too much demand.