Fix dirty state not matching save button state

[rchiodo]

Undo after save can make things go negative. This is fine as redo will push it back towards zero

[rchiodo]

Actually seems there's another issue. Updating a cell with new output and undoing doesn't work.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (ds/custom_editor@2fe009a). Click here to learn what that means.
The diff coverage is n/a.

@@                 Coverage Diff                 @@
##             ds/custom_editor    #9993   +/-   ##
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  Coverage                    ?   60.79%           
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  Files                       ?      564           
  Lines                       ?    29881           
  Branches                    ?     4492           
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  Hits                        ?    18165           
  Misses                      ?    10722           
  Partials                    ?      994

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[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities (and 0 Security Hotspots to review)
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No Coverage information
0.0% Duplication

[rchiodo]

Seems like VS code is doing weird stuff here too. Now at least my internal state matches theirs.

If you do this:

1. Edit file
2. Save file
3. Undo once - File tab is dirty at this point
4. Apply ANY random edit - File tab goes clean.

I think they need to change the API to allow us to set the undo stack and the dirty state.