Refactor classes in prep for perf improvements

[DonJayamanne]

Part of #7242
Starting notebooks and dealing with kernels is slow. Also, there'll be lot of work related to kernels in the near future.
Hence, splitting the two concerns into two separate classes.

This PR only focuses on the refactor (to make code reviews easy). I.e. no changes to logic/functionality in this PR

Later PRs will focus on refactoring the existing code to improve performance.

[codecov-io]

Codecov Report

Merging #8319 into master will decrease coverage by 0.29%.
The diff coverage is 57.56%.

@@            Coverage Diff            @@
##           master    #8319     +/-   ##
=========================================
- Coverage   59.33%   59.04%   -0.3%     
=========================================
  Files         504      506      +2     
  Lines       23096    23133     +37     
  Branches     3737     3733      -4     
=========================================
- Hits        13704    13658     -46     
- Misses       8500     8583     +83     
  Partials      892      892

Impacted Files	Coverage Δ
src/client/datascience/jupyter/kernelService.ts	54.67% <54.67%> (ø)
src/client/datascience/jupyter/notebookStarter.ts	59.34% <59.34%> (ø)
src/client/datascience/jupyter/jupyterExecution.ts	51.03% <87.5%> (-1.34%)	⬇️
src/client/testing/serviceRegistry.ts	48.64% <0%> (-46.85%)	⬇️
src/client/testing/codeLenses/main.ts	44.44% <0%> (-33.34%)	⬇️
...t/datascience/jupyter/jupyterDataRateLimitError.ts	66.66% <0%> (-33.34%)	⬇️
src/client/formatters/serviceRegistry.ts	75% <0%> (-25%)	⬇️
...ience/jupyter/jupyterDebuggerRemoteNotSupported.ts	75% <0%> (-25%)	⬇️
...cience/jupyter/jupyterDebuggerNotInstalledError.ts	60% <0%> (-20%)	⬇️
src/client/common/utils/icons.ts	83.33% <0%> (-16.67%)	⬇️
... and 7 more

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[rchiodo]

[IanMatthewHuff]