fix: match jest json output by making json reporter output ndjson-compatible

[bard]

Description

This change improves compatibility of the JSON reporter with Jest's --json output by making thhe reporter's output parsable as NDJSON.

Jest's JSON output is printed on a single line, which makes it consumable by NDJSON parsers when run in watch mode. Vitest's JSON output is pretty-printed; it can still be consumed when run in watch mode with e.g. stream-json, but that's more complex and less familiar for most. Given that this output is primarily intended for other programs, making it parsable as NDJSON makes intergrators' work easier. The occasional scenario of a human inspecting the output is covered by piping it into jq.

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