nix-cli: Add --json --pretty / --no-pretty

Default: istty(stdout)

This refactors `nix develop` internals a bit to use the `json` type
more. The assertion now operates in the in-memory json instead of
re-parsing it. While this is technically a weaker guarantee, we
should be able to rely on the library to get this right. It's its
most essential purpose.

tests/functional/meson.build

@@ -157,6 +157,7 @@ suites = [
       'impure-derivations.sh',
       'path-from-hash-part.sh',
       'path-info.sh',
+      'json.sh',
       'toString-path.sh',
       'read-only-store.sh',
       'nested-sandboxing.sh',