Improvements to startup of notebook

[DonJayamanne]

For #7242

Parallelise various parts of starting a notebook.

[codecov-io]

Codecov Report

Merging #8332 into master will increase coverage by 0.02%.
The diff coverage is 67.64%.

@@            Coverage Diff             @@
##           master    #8332      +/-   ##
==========================================
+ Coverage   59.39%   59.41%   +0.02%     
==========================================
  Files         508      508              
  Lines       23236    23255      +19     
  Branches     3750     3751       +1     
==========================================
+ Hits        13800    13817      +17     
- Misses       8540     8545       +5     
+ Partials      896      893       -3

Impacted Files	Coverage Δ
src/client/datascience/jupyter/notebookStarter.ts	65.65% <67.64%> (+6.31%)	⬆️
...atascience/interactive-window/interactiveWindow.ts	22.22% <0%> (-1.7%)	⬇️
src/client/terminals/codeExecution/helper.ts	21.27% <0%> (-0.73%)	⬇️
src/client/testing/navigation/symbolProvider.ts	93.1% <0%> (-0.65%)	⬇️
src/client/common/utils/localize.ts	93.99% <0%> (-0.02%)	⬇️
src/client/datascience/constants.ts	100% <0%> (ø)	⬆️
...client/datascience/jupyter/jupyterCommandFinder.ts	73.62% <0%> (+1.53%)	⬆️

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[rchiodo]

[IanMatthewHuff]