Adds mechanism to conditionally check EPUB

[breezykermo]

Rheo introduces a third supported target to Typst, EPUB. The target function in Typst allows the user to conditionally render based on the output, and we want that ability in Rheo for EPUB.

An important consideration is that one of Rheo's design goals is not to introduce new syntax, as we want a user to be able to take a file out of Typst and, in principle, compile it with the Typst toolchain instead. (We also certainly want all valid Typst files to be valid Rheo files.) EPUB compilation is, however, a feature that only exists in Rheo.

This PR implements EPUB target detection using Typst's inputs mechanism. Rheo sets sys.inputs.rheo-target to "epub", "html", or "pdf" during compilation via LibraryBuilder::with_inputs(). For user convenience, rheo also injects a target() polyfill during EPUB compilation:

#let target() = if "rheo-target" in sys.inputs { sys.inputs.rheo-target } else { std.target() }

This allows users to write natural Typst code like if target() == "epub" without any special imports or syntax in code that will be compiled by rheo.

For library developers: The target() polyfill is syntactic sugar that shadows the local function name. Packages that call std.target() (which is common practice to access the "real" target) will bypass the polyfill and see "html" (the underlying compilation target for EPUB). Library authors who want to support rheo's EPUB detection should check sys.inputs.rheo-target directly:

#let get-format() = if "rheo-target" in sys.inputs { sys.inputs.rheo-target } else { target() }

This pattern gracefully degrades in vanilla Typst, where sys.inputs exists but won't contain the rheo-target key.