Fix problem with undoing entire stack

[rchiodo]

If you save a file, then undo back to when the undo stack is empty, the file should be dirty.

VS code will show the dirty icon on the file, but we won't enable the save button on the UI.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities (and 0 Security Hotspots to review)
0 Code Smells

No Coverage information
0.0% Duplication

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (ds/custom_editor@97ad678). Click here to learn what that means.
The diff coverage is 100%.

@@                 Coverage Diff                 @@
##             ds/custom_editor   #10018   +/-   ##
===================================================
  Coverage                    ?    60.8%           
===================================================
  Files                       ?      564           
  Lines                       ?    29882           
  Branches                    ?     4491           
===================================================
  Hits                        ?    18171           
  Misses                      ?    10718           
  Partials                    ?      993

Impacted Files	Coverage Δ
...tascience/interactive-ipynb/nativeEditorStorage.ts	55.35% <100%> (ø)

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