gh-978: Add python.pipenvPath config setting.

[ericsnowcurrently]

(fixes #978)

• 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 #3957 into master will decrease coverage by 1%.
The diff coverage is 93%.

@@           Coverage Diff           @@
##           master   #3957    +/-   ##
=======================================
- Coverage      78%     78%   -<1%     
=======================================
  Files         406     406            
  Lines       18852   18861     +9     
  Branches     3025    3026     +1     
=======================================
+ Hits        14577   14578     +1     
- Misses       4273    4281     +8     
  Partials        2       2

Flag	Coverage Δ
#Linux	68% <65%> (ø)	⬇️
#Windows	68% <65%> (ø)	⬇️
#macOS	68% <65%> (ø)	⬇️

Impacted Files	Coverage Δ
src/client/interpreter/contracts.ts	100% <ø> (ø)	⬆️
src/client/common/types.ts	100% <ø> (ø)	⬆️
src/client/common/configSettings.ts	94% <100%> (+1%)	⬆️
...entActivationProviders/pipEnvActivationProvider.ts	79% <100%> (+6%)	⬆️
...ent/interpreter/locators/services/pipEnvService.ts	87% <80%> (ø)	⬇️
...tors/services/workspaceVirtualEnvWatcherService.ts	85% <0%> (-5%)	⬇️
src/client/common/process/pythonProcess.ts	89% <0%> (-3%)	⬇️
...rpreter/locators/services/baseVirtualEnvService.ts	86% <0%> (-2%)	⬇️
src/client/interpreter/helpers.ts	78% <0%> (-1%)	⬇️

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[DonJayamanne]

I'll be merging #3930 shortly, you'll need to modify src/client/common/terminal/environmentActivationProviders/pipEnvActivationProvider.ts file as well.

[ericsnowcurrently]

FYI, I updated the activation provider.