Explore reading flake info from Nix itself rather than CLI commands

[srid]

Instead of using nix flake show (and nix flake metadata) -- which reveal less information than we want -- consider adopting the devour-flake approach to inspect a flake from Nix itself, output a JSON and use that from Rust.

Examples of information lacking in CLI commands:

• meta.description is present, but meta.longDescription (markdown formatted long description) is not.
• Which packages (or devShells, checks, apps, etc.) are aliases of which (cf. packages.default = packages.emanote)

Brainstorming on custom metadata

(This is undecided)

• meta.highlight = true; for flakes having numerous packages, but want to highlighting the important ones. cf. nammayatri

[srid]

Let's use https://determinate.systems/posts/flake-schemas here

[srid]

Closing in favour of #248