Lerna -> nx, yarn -> npm

[demshy]

Summary
This PR moves the project from an outdated Lerna to nx. Also moving from yarn to npm, since using yarn is no longer necessary for monorepos and between updating yarn to v3 or changing to npm we preferred the latter.
Some notes:

• We entered beta state with Lerna so for versioning we will still use it (latest version though) and consider switching to changesets after the minor version release
• Website is not affected by this PR because moving it from yarn was not trivial - this should be done in a separate PR later on

These changes should not affect the product in any way, only the workflow of contributors. From my end it does seem much smoother - especially with nx's caching, building and testing should now be much faster.

Test plan

Checklist

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