This library is designed around supporting the kinds of use cases we
expect in the toolchain and other Carbon tools. It supports subcommands,
options, and prints help.
There is still a decent chunk of work to be done to finish polishing
this, but it should give us a solid starting point.
For details about the library and a brief roadmap, see the main comment
in `command_line.h` which provides a comprehensive overview of the
library and a roadmap of the remaining work.
Porting the driver to this went fairly well, but did require some
changes. That port is separated into a follow-up PR #2979.
---------
Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
Co-authored-by: Lucile Rose Nihlen <luci.the.rose@gmail.com>
Co-authored-by: Richard Smith <richard@metafoo.co.uk>
josh11b
