Add support for running an entire file whether or not it has cells

[rchiodo]

For #3362 - Make the run all cells command work whether or not we have cells.

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Has a news entry file (remember to thank yourself!)
• Unit tests & system/integration tests are added/updated
• Test plan is updated as appropriate
• package-lock.json has been regenerated by running npm install (if dependencies have changed)

[codecov]

Codecov Report

Merging #3938 into master will decrease coverage by 24%.
The diff coverage is 0%.

@@           Coverage Diff            @@
##           master   #3938     +/-   ##
========================================
- Coverage      78%     54%    -23%     
========================================
  Files         402     323     -79     
  Lines       18568   13488   -5080     
  Branches     2983    2096    -887     
========================================
- Hits        14350    7220   -7130     
- Misses       4216    5789   +1573     
- Partials        2     479    +477

Flag	Coverage Δ
#Linux	?
#Windows	?
#macOS	?

Impacted Files	Coverage Δ
...ient/datascience/editor-integration/codewatcher.ts	63% <0%> (-12%)	⬇️
...nittests/common/testVisitors/resultResetVisitor.ts	11% <0%> (-89%)	⬇️
...nt/unittests/pytest/services/testMessageService.ts	9% <0%> (-88%)	⬇️
...nt/unittests/common/services/testResultsService.ts	5% <0%> (-88%)	⬇️
...t/common/variables/environmentVariablesProvider.ts	13% <0%> (-87%)	⬇️
...ter/configuration/services/globalUpdaterService.ts	15% <0%> (-85%)	⬇️
src/client/common/process/proc.ts	13% <0%> (-82%)	⬇️
src/client/linters/pylint.ts	19% <0%> (-80%)	⬇️
src/client/unittests/pytest/runner.ts	18% <0%> (-80%)	⬇️
.../configuration/services/workspaceUpdaterService.ts	20% <0%> (-80%)	⬇️
... and 341 more

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[IanMatthewHuff]