Convert hds-lib from CJS to ESM output

- Add "type": "module" to package.json, compile to ESM via nodenext
- Remove js/ from git (now built on install via prepare script)
- Add .js extensions to all relative imports (nodenext requirement)
- Fix pryv CJS interop: default import + plugin pattern with (pryv as any)
- Rename webpack.config.js → webpack.config.cjs
- Convert all test files to ESM (import/export)
- Add all missing test files to browser-tests.js
- Add charset UTF-8 meta to browser-tests.html
- 204 tests passing in both Node and browser

Author: perki

.gitignore

@@ -1,4 +1,5 @@
 node_modules
+/js/
 coverage
 dist
 .gcd