Change the logic for when we show the "Install Python (Extension)" commands

[IanMatthewHuff]

Fixes #10583

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• Title summarizes what is changing.
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• Appropriate comments and documentation strings in the code.
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• Unit tests & system/integration tests are added/updated.
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[IanMatthewHuff]

Per good suggestion from @rchiodo. The old dummy command logic for when it showed I just ported directly to the new commands, but with the new commands we can improve that logic some.

Old Logic:
No python connections detected - extension uninstalled => suggest extension
No python connections detected - extension installed => suggest python

New logic:
File is python or undefined - extension uninstalled => suggest extension
File is python or undefined - extension installed - no python connections detected => suggest python

Restricts it to python or unknown files. And if you happen to have a globally detected python kernel already on this system this will still provide the extension install command in the picker to suggest that you can install python to detect more kernels and for a better experience.

[IanMatthewHuff]

Didn't need to be part of the class, and I needed it, so moved it out to a helper. Mild rename since the old name didn't seem quite accurate (it's more about getting the language of the document, not just the first cell as it prefers metadata over the first cell lang).

[codecov-commenter]

Codecov Report

Merging #10584 (bb0bb21) into main (521eda2) will increase coverage by 0%.
The diff coverage is 96%.

@@          Coverage Diff           @@
##            main   #10584   +/-   ##
======================================
  Coverage     71%      71%           
======================================
  Files        472      473    +1     
  Lines      28013    28025   +12     
  Branches    4697     4698    +1     
======================================
+ Hits       19943    19963   +20     
+ Misses      6196     6186   -10     
- Partials    1874     1876    +2     

Impacted Files	Coverage Δ
src/notebooks/languages/helpers.ts	88% <88%> (ø)
...oks/controllers/installPythonControllerCommands.ts	59% <100%> (+9%)	⬆️
src/notebooks/languages/cellLanguageService.ts	70% <100%> (+<1%)	⬆️
src/kernels/variables/kernelVariables.ts	56% <0%> (-2%)	⬇️
...ebooks/controllers/notebookIPyWidgetCoordinator.ts	88% <0%> (-1%)	⬇️
...ctive-window/editor-integration/codeLensFactory.ts	88% <0%> (-1%)	⬇️
src/kernels/raw/launcher/kernelProcess.node.ts	70% <0%> (-1%)	⬇️
src/kernels/execution/kernelExecution.ts	70% <0%> (-1%)	⬇️
src/kernels/common/baseJupyterSession.ts	78% <0%> (-1%)	⬇️
src/kernels/types.ts	100% <0%> (ø)
... and 7 more