compiler: wasm backend JSFFI sync exports

This commit implements the synchronous flavour of the wasm backend
JSFFI exports:

- `foreign export javascript "foo sync"` exports a top-level Haskell
  binding as a synchronous JS function
- `foreign import javascript "wrapper sync"` dynamically exports a
  Haskell function closure as a synchronous JS function
- `foreign import javascript unsafe` is now re-entrant by lowering to
  a safe ccall
- Also fix the issue that JSFFI dynamic exports didn't really work in
  TH & ghci (#25473)