jsonl is a basic format, where a file is a collection of jsons, where each json takes a line. This is common format for representing data in ML, where each line in .jsonl file represents a data frame. Considering, that the format is pretty primitive, I think, it is reasonable to support basic features for such file format out of the box. Basic features are:

• syntax colorization (basically same token types as in normal json)
• json icon in explorer/code tab (already there for some reason)
• add JSON Lines format for such files, so other extensions can potentially add features to this file format. The same way as there is a JSON with Comments format, for instance