feat(cmd,cli): In-house CLI library to replace zig-cli

[InKryption]

This introduces a CLI library developed in my spare time.
Some features to highlight:

• Data in & data out: unlike zig-cli, it isn't the executor, it simply parses a list of strings (typically argv[1..]), and returns a struct type specified by the caller, using normal fields to represent options, and subcommands as nested tagged unions.
• No global state: because it's data in & data out, no global state is required to share data between sub-commands.
• Statically verified: unlike zig-cli, there are no potential runtime bugs from an ill-defined command definition, all validation occurs at compile time with (mostly) informative error messages, and exhaustively specified associations.
• Easy sharing of options & groups of options across subcommands, with high capacity for unifying relevant information.
• Automatic help command output including default values, & automatically indented multiline help messages for options and sub-commands, with automatic tty support:
  

As a demo, this PR also adds some adjacent improvements to the cmd.zig code, such as removing the global variables, refactoring some of the help messages, and also happens to re-organize some of the options in certain ways that seemed slightly more logical.

There are more improvements that can be made to cmd.zig, in particular replacing much of config.Cmd with the newly-introduced option groups.

The cli.zig code is not totally perfect - it is perhaps a bit more complicated than need be, and it's not super obvious at a glance how different parts of the code flow into each other. I intend to simplify and improve these things, but the actual API should remain relatively the same.

[codecov]

Codecov Report

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