Overhaul the user interface

[nilsjor]

This PR will:

Fix minor issues after activating the environment

• Fix the problem where the shell prompt prefix interferes with the line-break in the terminal
• Clear the input history when activating

Make the package listing more pip-like

• Sort the installed and available packages alphabetically
• Extend the description for each supported planner
• Also make the name field more descriptive
• Add a homepage field for each supported planner
• Add the new command planutils show <package>
• Other minor improvements

Implement the shortname feature

Quoting from planutils/packages/README.md:

shortname specified in the manifest.json file will be used for the command-line invocation ...

• All planners with long, clunky names (e.g. forbiditerative-diverse-agl) get an optional shortname field in the manifest.
• This shortname can be used as an alternative for the sake of run, install, show etc.
• As specified, shortname will replace the case where run is omitted:
• For "quick" CLI invocations, it makes sense to keep it short
• For scripting, the full name can still be used in conjunction with planutils run ...

[haz]

I think we'll run into name clashes, and preferences from package authors (e.g., I think the Basel crew urged that downward be used instead of fd), but all those can be addressed as separate one-off discussions. This PR brings a lot to the table -- thank you so much, @nilsjor !!