Ensure launchingFile exists when setting working directory

[joyceerhl]

For #6549

With fix, interactive window now reflects working directory of the launching file:

@DonJayamanne I saw that this line actually used to be launchingFile && await this.fs.localFileExists(launchingFile) and you changed it to launchingFile && (await this.fs.localDirectoryExists(path.dirname(launchingFile)) in https://github.com/microsoft/vscode-jupyter/pull/5911/files#diff-a0dd7eee8719435e87b786dfb43d089e73561a33ce842613a2fc7a3894bea959R968

However relying on path.dirname causes problems with setting the current working directory for the interactive window and I would argue that we should always be checking whether the launchingFile is valid, since otherwise it's possible for us to end up in states like this: #6549 (comment)

Do you foresee any issues with this change?

[codecov-commenter]

Codecov Report

Merging #6577 (93f4e28) into main (f287670) will increase coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main   #6577   +/-   ##
=====================================
  Coverage     69%     69%           
=====================================
  Files        410     410           
  Lines      28243   28240    -3     
  Branches    4193    4196    +3     
=====================================
+ Hits       19632   19702   +70     
+ Misses      6960    6862   -98     
- Partials    1651    1676   +25     

Impacted Files	Coverage Δ
src/client/datascience/jupyter/jupyterNotebook.ts	76% <ø> (-1%)	⬇️
src/client/datascience/notebook/introStartPage.ts	58% <0%> (-42%)	⬇️
src/client/datascience/jupyter/jupyterSession.ts	74% <0%> (-10%)	⬇️
...t/datascience/kernel-launcher/localKernelFinder.ts	84% <0%> (-6%)	⬇️
src/client/common/process/pythonDaemonPool.ts	75% <0%> (-5%)	⬇️
.../datascience/kernel-launcher/remoteKernelFinder.ts	90% <0%> (-4%)	⬇️
...rc/client/common/process/pythonExecutionFactory.ts	82% <0%> (-3%)	⬇️
...ent/datascience/jupyter/pythonVariableRequester.ts	72% <0%> (-2%)	⬇️
.../datascience/notebook/notebookControllerManager.ts	81% <0%> (-2%)	⬇️
...ent/datascience/jupyter/kernels/kernelExecution.ts	67% <0%> (-2%)	⬇️
... and 24 more