feat: Write cappa backend/parser.

[DanCardin]

• Adds native cappa parser option vs argparse
• Renames render option to "backend"
• Collapse separate Subcommands abstraction into Subcommand
• Renames Subcmd to Subcommands
• Adds support for autocompletion when using new parser
• Removes "rich" extra
  • Adds rich as a required extra dependency
    • It is core to custom/colored help formatting for new parser, and the now more-unified stdout/stderr output.
  • rich-argparse needs to now be installed separately, if used
• Adds a new Output implicit dependency option
• Adds new theme option (currently only applied to custom parser)
• Adds group option to Arg to control how arguments are grouped together in help text
• Adds hidden option to Arg, to hide arguments from help text.
• Ensures help= applies to all levels of subcommands
• Fixes Confusing/unclear parsing output #2

[coveralls]

Pull Request Test Coverage Report for Build 6462821733

• 685 of 685 (100.0%) changed or added relevant lines in 17 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.05%) to 99.951%

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Files with Coverage Reduction	New Missed Lines	%
src/cappa/argparse.py	1	99.5%

Totals
Change from base Build 6355604095:	-0.05%
Covered Lines:	1452
Relevant Lines:	1453

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💛 - Coveralls