Add `if target() == "epub"` support

[breezykermo]

There is currently no way to differentiate between HTML and EPUB output in a typst file, as upstream typst does not yet support EPUB. We should thus add support for an if target() == "epub", which I think is fine for at least two reasons:

1. It will still be valid typst, and so will still compile. (It will just always evaluate to false, as the typst compiler doesn't yet support that target.)
2. Support for this statement is likely to be added in upstream typst, as EPUB is on the roadmap as a target.

This will allow us to make the rheobook template a lot nicer for EPUB.

[breezykermo]

Closed by #68