Build extension on every PR

[astrojuanlu]

Could make testing easier.

This gives some hints https://code.visualstudio.com/api/working-with-extensions/continuous-integration#github-actions but I guess it would need

• A versioning scheme that reflects the commit to avoid going crazy
• A way to store the vsix file as an artifact so it can be downloaded
• (Bonus points) An automated comment that attaches the extension, inspired by Bump ws from 7.5.9 to 7.5.10 kedro-website#192 (comment)

[astrojuanlu]

@noklam remembered that VS Code extensions don't have the concept of pre-releases yet https://code.visualstudio.com/api/working-with-extensions/publishing-extension#prerelease-extensions

Artifacts on GHA https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/storing-and-sharing-data-from-a-workflow

Another option could be running it on Gitpod (maybe related: https://www.gitpod.io/docs/flex/introduction)