fix: include LICENSE in VSIX package

[stellhub]

Summary

• allow the root LICENSE file through .vscodeignore
• ensure the packaged VSIX includes the extension license file

Closes #958

Testing

• npm test
• npm run package
• verified the generated VSIX now contains LICENSE.txt

[stellhub]

Thanks for taking a look. This PR comes from a fork, and GitHub Actions is currently blocked with an action_required status before any jobs can start. I have already run the relevant checks locally:\n- npm test\n- npm run package\n- verified the generated VSIX now contains LICENSE.txt\n\nCould a maintainer please approve and run the workflows for this PR?