Add long_desc and split -h from --help

[aaronmallen]

Commands and namespaces can now set a long description with long_desc. -h prints the
short description from desc; --help prints the long one. This mirrors how Clap treats
about and long_about in Rust.

class Echo < Dry::CLI::Command
  desc "Prints given input"
  long_desc <<~DESC
    Prints the given input back to standard output.

    When no input is given, it prints an empty line.
  DESC
end

$ foo echo -h shows "Prints given input". $ foo echo --help shows the full text.

How

• Command and Namespace gain a long_desc class method that stores @long_description.
• Parser registers -h and --help as separate handlers and marks the result when the
  user asked for long help.
• Banner picks the description by help mode: long help uses long_description and falls
  back to description; short help always uses description. Multi-line text now indents
  under the Description: heading, and blank lines stay blank.
• Command listings still use the short desc.

Compatibility

Commands without long_desc print the same output for both flags, so nothing changes for
existing CLIs. All 328 existing specs pass untouched; six new specs cover the split, the
fallback, and namespaces.