Extend target validation to also check the proper existence of the (prejob|postjob)execution target

[jessehouwing]

The current validation checks the existence of the file referenced in the execution.target:

https://github.com/microsoft/tfs-cli/blob/master/app/lib/jsonvalidate.ts#L135C3-L158C4

But it doesn't check the prejobexecution.target not the postjobexecution.target. These are much less used execution options, but it would be nice to validate these too.

The current functionality can easily be wrapped in a similar look as used here:

https://github.com/microsoft/tfs-cli/blob/master/app/lib/jsonvalidate.ts#L67-L75