feat: deeply compare diagnostics and simplify build orchestration

[Vtec234]

This PR reduces infoview re-renders (1.) and simplifies the build setup (2.) Fixes leanprover-community/ProofWidgets4#176.

1. InfoAux fetches goals and widgets whenever the Object.is-identity of lspDiagsHere changes. The React effect that sets lspDiagsHere tries to maintain its identity, but it used to do so by comparing LeanDiagnostics for ===-identity. We now use react-fast-compare-identity. We do not try to memoize earlier in the data flow (e.g. when setting LspDiagnosticsContext) in any principled way.

   The ===-comparison was never correct - fresh diagnostic objects are created on every publishDiagnostics - but the issue was primarily exposed by feat: 'unsolved goals' & 'goals accomplished' diagnostics lean4#7366 which produces a hidden diagnostic on every declaration, so that lspDiagsHere is recomputed at most positions.

2. We remove Lerna, leaving only Nx (which Lerna uses under the hood). This unbreaks building in a worktree folder nested within the main repo folder (vscode-lean4/worktree-my-branch/). This also drops ~400 transitive dependencies and should otherwise be functionally equivalent.