Use Lerna 6 with pnpm instead of yarn workspaces

[web-padawan]

Describe your motivation

We had a few cases when some npm dependency was missing:

• fix: add missing Lumo and Material dependencies #4355
• fix: add missing field-base dependency #4627

This is a known issue of Lerna known as "phantom dependencies".
It comes from the way how bootstrapping works when using Lerna + Yarn workspaces.

Describe the solution you'd like

Consider using Lerna 5.5.0 that added pnpm support for workspaces.

One thing that pnpm provides with its symlinked module structrue (if you don't use --shamefully-hoist) is that it isolates modules well, so there is no phantom dependency problem. We need to check if this feature can be used with Lerna.

[web-padawan]

Prototype branch with Lerna 6 + pnpm: https://github.com/vaadin/web-components/tree/pnpm

Summary

Confirmed that this setup reports all the missing packages when running tests with web-test-runner.
Based on this research, the following PRs have been submitted to address various findings:

• fix: add missing Lumo and Material dependencies #4728
• refactor: use relative imports in component-base #4729
• chore: add missing Lit dev dependency for tests #4730
• chore: add imported packages to direct dependencies #4731
• chore: fix components dev dependencies used in tests #4736
• test: update visual tests to not use legacy adapter #4738
• refactor: use relative import in menu-overlay mixin #4740
• chore: add missing icon dev dependency used in tests #4741
• chore: add missing dev dependencies used in tests #4744
• chore: add component-base dependency used in tests #4745
• chore: add missing dependency to integration tests #4746

One remaining problem when using pnpm is test/typings folder that imports legacy component packages.
We should remove that folder completely, alongside with removal of deprecated packages in Vaadin 24.

Conclusion

I propose to implement pnpm support after 24.0 is out, so that we no longer need to maintain 23.2 branch.
Otherwise, we would have to manually backport some of the PRs listed above (missing dependencies).

See also

• Lerna docs: https://lerna.js.org/docs/recipes/using-pnpm-with-lerna
• pnpm CI docs: https://pnpm.io/continuous-integration#github-actions