Document JavaScript port (jslt-js) in README; add it to playground

[amkraev697642]

Summary

• README: link to jslt-js (npm, Node + browser, passes upstream conformance tests).
• Playground: adds Run in JS, Run (Java server) unchanged.

Notes

• No changes to core Java engine.
• Playground tested locally with Java 11

[amkraev697642]

@larsga — Please review when you have a moment. I found JSLT very useful, so when I found there is no javascript engine available, faithfully ported it. Most direct port, 1 to one class files. All fixture tests pass + added more tests:

➜ jslt-js git:(main) ✗ npm test

jslt-js@0.1.2 test
node --test

✔ fallback (macro): short-circuits on first value that isn't null/{}/[] (isValue, not isTrue — "" and 0 count) (2.526833ms)
✔ flatten (3.116458ms)
...
✔ custom JS functions register via the extensions/functions option (0.400291ms)
✔ parse errors surface as JsltException with location info (0.476167ms)
ℹ tests 710
ℹ suites 0
ℹ pass 710
ℹ fail 0
ℹ cancelled 0
ℹ skipped 0
ℹ todo 0
ℹ duration_ms 457.871834

This PR just adds README link to jslt-js, and a Run in JS button in the playground.