This commit refactors the wasm dyld script so that it can be used to
load and run wasm shared libraries fully client-side in the browser
without needing a wasm32-wasi-ghci backend:

- A new `DyLDBrowserHost` class is exported, which runs in the browser
  and uses the in-memory vfs without any RPC calls. This meant to be
  used to create a `rpc` object for the fully client side use cases.
- The exported `main` function now can be used to load user-specified
  shared libraries, and the user can use the returned `DyLD` instance
  to run their own exported Haskell functions.
- The in-browser wasi implementation is switched to
  https://github.com/haskell-wasm/browser_wasi_shim for bugfixes and
  major performance improvements not landed upstream yet.
- When being run by deno, it now correctly switches to non-nodejs code
  paths, so it's more convenient to test dyld logic with deno.

See added comments for details, as well as the added `playground001`
test case for an example of using it to build an in-browser Haskell
playground.