This commit adds JavaScript util code to utils to support the wasm
backend's JSFFI functionality:

- jsffi/post-link.mjs, a post-linker to process the linked wasm module
  and emit a small complement JavaScript ESM module to be used with it
  at runtime
- jsffi/prelude.js, a tiny bit of prelude code as the JavaScript side
  of runtime logic
- jsffi/test-runner.mjs, run the jsffi test cases

Co-authored-by: amesgen <amesgen@amesgen.de>