Add functionality to display the default help message when running
spruce without any arguments or when the help flag is used.
Customise the help message for the subcommands.
Additional changes:
- Refactor: move the Runner interface to the internal cmd package
- Fix: Add a summary for each of the subcommands.
- Refactor: Use string builder to replace string literals.
- Perf: Use a switch statement to only create the subcommand that the
user calls.
