refactor: replace ndjson with jsonl specification

[byCedric]

Linked issue

There are two (slightly) different standards for Newline-Delimited JSON; ndjson and jsonl.

The main difference between these two are around the allowed content characters within the JSON files:

• ndjson - 3.1. Serialization

  Each JSON text MUST conform to the [RFC8259] standard and MUST be written to the stream followed by the newline character \n (0x0A). The newline character MAY be preceded by a carriage return \r (0x0D). The JSON texts MUST NOT contain newlines or carriage returns.

  All serialized data MUST use the UTF8 encoding.

• jsonl - 1. UTF-8 Encoding

  JSON allows encoding Unicode strings with only ASCII escape sequences, however those escapes will be hard to read when viewed in a text editor. The author of the JSON Lines file may choose to escape characters to work with plain ASCII files.

  Encodings other than UTF-8 are very unlikely to be valid when decoded as UTF-8 so the chance of accidentally misinterpreting characters in JSON Lines files is low.

Since we embed code in these stats files, which may or may not contain (un)escaped UTF-8/UTF-16 characters and likely contains JSON-escaped newlines (which makes it hard to parse JSON by partial line chunk), we don't fully match the ndjson spec.

On top of that, .jsonl seems to be a recognized format in vscode. So fully swapping over to jsonl and using .jsonl extension seems to be a low hanging fruit.