fix: fix workspace-filtered install with linked strategy

[owlstronaut]

The isolated tree created by _createIsolatedTree() used Arrays and plain objects incompatible with workspace-filtered installs. This caused crashes when using --workspace with --install-strategy=linked, broken since the linked strategy was introduced in npm 9.4.0.

• Use Map/Set for children, fsChildren, and inventory to match real Node tree interfaces
• Add workspaces Map, workspace Link entries in children, and a working inventory.query('realpath', ...) implementation
• Fix diff.js filterSet builder to visit link targets so store entries are included in workspace-scoped diffs
• Skip re-rooting for linked strategy (filterSet references don't survive the tree-swap)
• Remove dead .get && guard on children (now a proper Map)

Fixes: #9038

[owlstronaut]

@manzoorwanijk Could you give this a try?

[manzoorwanijk]

Gave it a test and it works great. Thanks.