Add `--format=json` option

The default format, which is `pkgconf` compatible is useful for pkg-config, but also mixes various kinds of flags together on a single line. A JSON format could also be provided for tools willing to implement cps-config specific support, which would allow separating different kinds of flags. This would allow a build system to run cps-config once to get all of the information it needs, but be able to separate that information internally without post processing or running pkg-config/pkgconf multiple times with different flags.

One example where this may be important is separating the output of `--cflags-only-I` and `--cflags-only-other`

Start at the command-line handling for the existing format options and trace how cflags and other flag categories are collected. Define the JSON output structure for those categories and add support for --format=json; done means one invocation exposes separable flag information without post-processing.

Written by the indexing model from the issue text.