ci(release_lsp): fix vscode extension build

[nhedger]

Summary

When creating #219, we forgot to update the release_lsp workflow to use pnpm instead of npm.

This PR should solve the issue.

• Setup pnpm in GitHub Actions
• Update npm with pnpm in scripts

Test Plan

• Trigger the pipeline manually (workflow_dispatch) to ensure that it behaves as intended (@ematipico).

[ematipico]

For security reasons, I can't trigger a workflow from a fork, although you can try it in your own fork :)

We just need to make sure that it builds

[ematipico]

Just tested it, still some issues. This time, when packaging the extension.

[nhedger]

I've identified the issue. I'll pr something shortly.