ENH: Create CLI Extension Wizard 'launcher'

Create a wrapper script to set up the environment (using the Slicer
launcher's --launcher-show-set-environment-commands) and execute the CLI
Extension Wizard via the appropriate Python interpreter. (Also, install
the Python application launcher wrapper for the same.)

This should provide a single, consistent mechanism for running the CLI
Extension Wizard (from a bash shell) from both build and install trees,
and regardless of whether system or Slicer-built Python is used. (On
Windows, git-bash must be used.)