Change rawjson progress to one line per JSON record?

[felixfontein]

This would make it easier to parse the raw JSON output, since you can simply split by newlines and feed every line into a JSON parser.

Right now the JSON output comes "nicely" formatted for human consumption, but harder to parse automatically (this was added in #4113 (comment)).

My main intention is to make this easier to parse when appearing in docker compose output (see docker/compose#11478). Since docker compose stdout output can also contain stdout from other sources than buildkit, it's a lot easier to parse stdout when every JSON record is in its own line (and just a single line).

[ndeloof]

this would align with docker commands which produces newline-delimited JSON stream

[felixfontein]

I created #4770 to change this.