Add AMD distribution path for OJET/RequireJS compatibility (#33)

[miquelgall]

Summary

• add dual Rollup outputs for ESM and AMD artifacts
• expose AMD entrypoint via package subpath export (@ofs-users/plugin/amd)
• document OJET/RequireJS integration and path mapping guidance
• add compatibility tests for package metadata and AMD artifact config
• switch test script to Node built-in test runner (node --test) so tests run without extra dev dependency setup

Changes

• rollup.config.mjs
  • output now includes:
    • dist/ofs-plugin.es.js (format: es)
    • dist/ofs-plugin.amd.js (format: amd)
• package.json
  • add exports[.] with ESM default and types
  • add exports[./amd] pointing to ./dist/ofs-plugin.amd.js
  • update test script to node --test
• README.md
  • add OJET/RequireJS compatibility section, module IDs, and mapping example
• test/general/compatibility.test.js
  • validate ESM defaults + AMD subpath export
  • validate Rollup config includes AMD artifact output

Validation

• npm test (passes)
• npm run prepare (generates dist/ofs-plugin.es.js and dist/ofs-plugin.amd.js)

Closes #33

[btoron]

Closes #25 also