Ensure using request: launch item in launch.json for debugging sends pathmappings

[karrtikr]

For #3568

• 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!)
• [ ] Appropriate comments and documentation strings in the code
• [ ] Has sufficient logging.
• [ ] Has telemetry for enhancements.
• 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)
• [ ] The wiki is updated with any design decisions/details.

[DonJayamanne]

I don't see any changes to package.json that would enable pathMapptings in launch.json for launch types.
I.e. from a users perspective this is still broken.
Also, in light of Remote Extensions I'm not sure we need this, will need to discuss with team.

[DonJayamanne]

Please add the following change for launch configuration:

• If debugConfiguration.pathMappings is empty, then always add the following as a default value:

                {
                    "localRoot": "${workspaceFolder}",
                    "remoteRoot": "."
                }