Add Miso.DSL, drop jsaddle.

[dmjio]

This PR is a big one. It replaces jsaddle w/ our own JS FFI DSL (in IO). It also creates raw FFI for interacting w/ the WASM backend along with a Data.JSString API (interface parity with Data.Text).

• Removes jsaddle and JSM (moves everything to IO).
• Creates ffi/ sub-directory for the 3 backends.
• Creates Data.JSString for WASM backend w/ it's own JSString Data.Text API.
• Creates Miso.DSL in IO, uses jsaddle-like combinators.
• Uses true synchronous callbacks to establish component mounting (syncCallback').
• Adds From/ToJSVal marshalling logic (w/ ToObject, ToArgs classes).
• Related GHC PR: Adds ability for WASM browser mode to host static assets here
• Upstreams @amesgen work from jsaddle-wasm to construct a raw FFI statement in TH (workaround for js-sources)
• mount() function is now written synchronously, adjust spec/ files accordingly
• Drop jsaddle from nix/ scripts.
• Drops conflicting (#>) combinator
• Updates JS and vanilla GHC backends to have parity w/ WASM backend impl in ffi/
• Add tests
• Update haddocks

Partial contingency on https://gitlab.haskell.org/ghc/ghc/-/merge_requests/15251 to maintain parity with jsaddle-warp features.