Support `pkl` previews

[macintacos]

Right now if you're previewing a .pkl file (https://pkl-lang.org/index.html), it seems to treat it like a binary file:

Not entirely sure how to influence that, but it'd be nice to treal them like any other file (neovim seems to be able to render those files perfectly fine).

[dmtrKovalenko]

@gustav-fff remove the .pkl from the is_known_binary_extension binary extension classification as well as .pickles probably as this is the same framework idk if someone still using it

[gustav-fff]

[triage-bot] LINKED PR: #552 (open) — #552

Removes pkl and pickle from is_known_binary_extension in crates/fff-core/src/file_picker.rs. .pkl collided with Python pickle; Pkl config files are plain text so preview now renders as text with syntax highlighting. One-line change, awaiting maintainer review.

Honk-Honk 🪿