Avoid blocking queued syncs on stale save errors

[mmtftr]

Summary

• make Jupytext error notifications non-blocking so a failed save does not stall later queued syncs
• suppress sync error output for stale save tasks when a newer save for the same file already exists
• keep the existing queue behavior otherwise unchanged
• resolves Saving back to back error: jupytext.cli.SynchronousModificationError #37

Problem

When a sync triggered by onDidSaveTextDocument failed, the extension awaited
vscode.window.showErrorMessage(...). That kept the task promise unresolved and prevented the next queued
sync for the same file from starting.

Rapid consecutive saves also produced error output for obsolete sync attempts, even when a newer save had
already superseded the failing task.

Fix

• stop awaiting the error popup in runJupytext
• assign each save-triggered sync a monotonically increasing per-file version
• only surface sync errors if the failing task is still the latest version for that file

Verification

(node 24.11.1)

• npm run compile
• npm run lint

[caenrigen]

Thanks for debugging it and the suggested fix

I will have to look at this into more detail when I find a bit of time, it touches on functions that are used for a few different purposes beside the sync