Multiple bibliography citations

[Omikhleia]

(Split from #135)
So we got "basic" citation done in #135

But as stated there, this only works as intended with single citations [@key, locator]

To support multiple citations properly, i.e. [@key1, locator1; @key2, localtor2] we first need sile-typesetter/sile#2196 - and then use here whatever low-level command is made available to us.

[Omikhleia]

Waiting for sile-typesetter/sile#2230 to adapt here.