Scaffolding command for new built-in and custom rules

[anderseknert]

Something like:

regal new rule --type custom --category naming --title lower-case

Which would:

1. Create a .regal directory if none exists.
2. Create a .regal/rules directory if none exists.
3. Create a .regal/rules/category/title.rego file pre-populated with the basic required policy structure.
4. Create a .regal/rules/category/title_test.rego file pre-populated with the basic required policy test structure.

And something similar to the above for creating new built-in rules as well, where the new files are added to the Regal bundle instead, and where there's also an entry added for documentation in the docs directory.