What does this MR do?

this is not exactly great, but it should be "good enough" (cc @sugar)

note that the new group function should not escape in the wild, as we don't document it and only use it internally

I tried using $[scale foo bar] instead of $[group foo bar], but that would be rendered as <i>foo bar</i> when sent over the network to non-misskey instances, and we don't want that