Debugger: support "Input variables" from launch.json

[czeidler]

See "Input variables" section in https://code.visualstudio.com/docs/reference/variables-reference

This is, for example, useful in a mono repo to pick the package path for a debug action. For example, you select "Run tests" and then you are prompted by a menu for choosing the particular test to be run.

Could be implemented as another step in the launch dialog?