fix: Build frontend package before publishing to npm

[feanil]

Summary

• The @openedx/sample-plugin package was shipping raw JSX (main: src/index.jsx), which webpack in MFE builds cannot process since babel-loader excludes node_modules by default
• Follows the same pattern as @openedx/frontend-plugin-framework: compile src/ → dist/ with fedx-scripts babel before publishing
• Updates main to dist/index.js and adds files: ["dist"] so only compiled output is published to npm

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.23%. Comparing base (c8c3634) to head (7d38530).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #27   +/-   ##
=======================================
  Coverage   88.23%   88.23%           
=======================================
  Files          13       13           
  Lines         408      408           
  Branches       16       16           
=======================================
  Hits          360      360           
  Misses         40       40           
  Partials        8        8           

Flag	Coverage Δ
unittests	88.23% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.