Add option to track and restore open files per workspace

[jsmorabito]

Summary

Carries forward the feature from #117 (Chiarchiaooo:master), rebuilt on top of current master. That branch's diff was unmergeable through GitHub directly — it forked from a 2021 snapshot of this repo and was never synced, so GitHub reported it as 2,813 additions / 521 deletions across 24 files and marked it CONFLICTING, none of which reflected the actual change. This PR cherry-picks the same 4 commits (original authorship preserved) cleanly onto current master.

• Adds a trackOpenFiles setting (default on) that remembers which file was open in each pane per workspace and restores it when switching back
• Utils.captureOpenFiles/restoreOpenFiles reuse the existing split/tabs/leaf traversal pattern already used by setChildId
• Restoration runs before the existing "File Overrides" feature, so explicit per-leaf overrides still take precedence
• Handles a tracked file having been deleted/moved by clearing the leaf instead of leaving a dangling reference

Closes #117

Test plan

• Cherry-picked cleanly onto current master (one trivial auto-merge, no real conflicts)
• npm run build and tsc --noEmit clean
• Reviewed interaction with existing File Overrides feature and Workspace Modes — no conflicts, Modes don't touch the main split so this feature doesn't apply to them

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