Add scarb cache path command

[mkaput]

Problem

Currently, the only way to know where Scarb puts its cache files is to look at relevant Scarb documentation and manually implement if-else chain based on running platform. For example, that's what setup-scarb action does. It would be much easier for everyone if global paths could be accessible programmatically from scarb CLI.

Proposed Solution

Add a new command scarb cache path (residing next to scarb cache clean) which will just println! cache path from Config.

CLI help message for this command: Print path to Scarb's global cache directory

Notes

• For extra reference, have a look at yarn cache dir.
• Implementation is very trivial, please add a test which just ensures this command succeeds and outputs a single line.
• Bonus points for integrating this into setup-scarb