Bump clap version to 4.0 and switch to the Derive API

[tranzystorekk]

Hi, hope this isn't too big for a first contribution 🙂

Should make the code a bit more readable with the Derive API.

Notes:

• had to bump pest in Cargo.lock to resolve a dependency conflict
• the extension flag now additionally accepts comma-delimited lists, e.g. -e strikethrough,table,footnotes
• clap no longer adds author info by default, so I skipped it for now (adding it back is a bit involved)

[tranzystorekk]

Seems we should somehow disable clap for wasm: lambda-fairy/rust-errno#21 (comment), or at least the wrap_help feature

[kivikakk]

Thank you so much, this is perfect, and certainly not too big at all! Really clear changes and very easy to review. Massively appreciated! 🥰