--reporter=ndjson should only log json

[vjpr]

Currently console.log messages in pnpmfile will be printed, and also usage information.

When json is enabled, all output should be in json format. Maybe just with a {type: 'stdout', ...} would be good.

Otherwise, line.startsWith('{') must be used to check every line of the output.

It would also be nice to allow multiple reporters...such as stdout, and the ndjson redirected to a file.