feat: add unformatted-count and unformatted-files outputs

[dsherret]

Adds two outputs so a workflow can act on the result without parsing the log:

• unformatted-count: the number of files that aren't formatted
• unformatted-files: the files that aren't formatted, one per line, relative to the workspace

The check step now always runs dprint check --json when node is available and the script records the outputs. The annotations input only controls whether the ::error lines are emitted, so the outputs are available with annotations disabled too. The outputs are empty on dprint before 0.57 or without node, like annotations.

The cache job in CI asserts the outputs for both the failing check (1 and poorly-formatted.json) and the passing one (0 and empty).