feat: add support to preview typst targeting html

[Myriad-Dreamin]

Adding support to previewing typst document in HTML target, i.e. typst compile main.typ main.html. Only PoC works, but the arch is already set up.

Although different targets share a same main source file, when targeting html, typst interpreter runs on completely different control flow from the one targeting paged. For example, the following document has content html.div(y) when targeting html, and has content box(width: 10em, x) when targeting paged:

#context if page.target == paged {
  box(width: 10em, x)
} else {
  html.div(y)
}

A main question is throwed at once: how do we inform the language server to run code targeting html? Or whether we should run both targets to get diagnostics on both targets as many as possible.