Support JSON lines output

It would be nice if the was an option to parse output from tools that output JSON lines[^1]. For example Selene[^2] is such a tool:

```console
$ selene --display-style json2 --no-summary init.lua
...
{"type":"Diagnostic","severity":"Warning","code":"unscoped_variables","message":"`vim` is not declared locally, and will be available in every scope","primary_label":{"filename":"init.lua","span":{"start":11315,"start_line":339,"start_column":0,"end":11318,"end_line":339,"end_column":3},"message":""},"notes":[],"secondary_labels":[]}
{"type":"Diagnostic","severity":"Error","code":"undefined_variable","message":"`vim` is not defined","primary_label":{"filename":"init.lua","span":{"start":11477,"start_line":346,"start_column":0,"end":11480,"end_line":346,"end_column":3},"message":""},"notes":[],"secondary_labels":[]}
{"type":"Diagnostic","severity":"Error","code":"undefined_variable","message":"`vim` is not defined","primary_label":{"filename":"init.lua","span":{"start":11605,"start_line":349,"start_column":0,"end":11608,"end_line":349,"end_column":3},"message":""},"notes":[],"secondary_labels":[]}
{"type":"Diagnostic","severity":"Warning","code":"unscoped_variables","message":"`vim` is not declared locally, and will be available in every scope","primary_label":{"filename":"init.lua","span":{"start":11605,"start_line":349,"start_column":0,"end":11608,"end_line":349,"end_column":3},"message":""},"notes":[],"secondary_labels":[]}
...
```

Something similar to `errorsRoot` I guess, but `jsonLines: true|false`.

[^1]: https://jsonlines.org/
[^2]: https://github.com/Kampfkarren/selene

No files or tests are named. Start by tracing the existing errorsRoot option and the linter-output parsing entry point, then compare the expected Selene JSON-lines records with the current parser. Done means a jsonLines option can parse one diagnostic object per line and expose the resulting diagnostics without breaking existing output formats.

Written by the indexing model from the issue text.