fix(arborist): symlink workspace file: deps on non-workspace local packages

[manzoorwanijk]

In continuation of our exploration of using install-strategy=linked in the Gutenberg monorepo, which powers the WordPress Block Editor.

With install-strategy=linked, a workspace declaring a file: dependency on a local package that is not itself a workspace was silently skipped.
npm install reported up to date, created no node_modules entry, and the workspace could not resolve the dependency at runtime, all with no error or warning.
This covered two variants: a target inside the repo root but outside the workspaces globs, and a target outside the repo root entirely (the real npm --workspace=<ws> link <external> case).

The cause is in #assignCommonProperties in isolated-reifier.js, where each dependency is classified as local (symlinked directly) or external (extracted into .store).
The check n.isWorkspace || node.fsChildren?.has(n) only recognized file: targets via fsChildren, which exists on the root ideal tree but not on workspace nodes, and never contains targets outside the repo root.
A workspace to workspace file: dep still worked because the target satisfies n.isWorkspace, but a non-workspace or out-of-repo target failed both checks, was misclassified as external, and had no store entry, so #processDeps dropped it with no symlink.

The fix detects a file: dependency from its edge: a file: dep surfaces as a Link edge whose resolved spec starts with file:.
Classifying those targets as local handles both variants uniformly, regardless of fsChildren membership, and reuses the workspace proxy already created for the target in makeIdealGraph.
Only file: link edges and workspaces are treated as local, so no registry dependency can be misclassified.

References

Fixes #9589
Related #9115, #9166, #9210

[github-actions]

🎉 Backport to release/v11 created: #9593