Adds js-exec, a sandboxed JavaScript/TypeScript runtime powered by QuickJS (WASM). (#122)

* version

* Introduce JS execution

* Full fetch

* Align fs sync cases

* Stabilize post merge

* Throw helpful errors on node.js compat errors

* Update error message to point to --help

* validate

* Better timeout handling and span execs

* Address feedback

* Make a 1st class feature for args forwarding

* QuickJS hardening

* Feedback

* Readme edits

* Address feedback