Fix order of cells inserted

[DonJayamanne]

Cells weren't inserted into the model in the right order.

• Insert cell using a
• Insert cell using b
• Save NB and reopen
• All the new cells are at the very bottom

Added https://github.com/microsoft/vscode-python/issues/10230 and moved into backlog and assigned to ds_editor epic. I think in light of this issue it must be done. Else it'll be bad for users to report this.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (ds/custom_editor@ca10f93). Click here to learn what that means.
The diff coverage is n/a.

@@                 Coverage Diff                 @@
##             ds/custom_editor   #10229   +/-   ##
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  Coverage                    ?   60.83%           
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  Files                       ?      567           
  Lines                       ?    30586           
  Branches                    ?     4363           
===================================================
  Hits                        ?    18608           
  Misses                      ?    11031           
  Partials                    ?      947

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[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!

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[DonJayamanne]

Force merging as branch is broken due to CI/GitHub issues (wrong jobs were flagged as Required).