Add --json-help argument

[natebosch]

Add a hidden --json-help argument which writes the usage information
in JSON Schema format.

[github-actions]

PR Health

Changelog Entry ❗

Package	Changed Files
package:test_core	pkgs/test_core/lib/src/executable.dart pkgs/test_core/lib/src/runner/configuration.dart pkgs/test_core/lib/src/runner/configuration/args.dart pkgs/test_core/pubspec.yaml

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[natebosch]

This is using an unpublished feature in package:args.

dart-lang/core#897

[natebosch]

Opening this PR to have the code in a shared place. Not sure if this is going to be worth landing (or when) but this branch can be used to generate the schema.