feat: support clickable relative filepaths

[bashbunni]

This is generally supported by markdown readers to include a relative filepath (e.g. [read more](./more-info.md)) which provides a clickable link to open the other file.

Tested with Joplin and Neovim Markdown Reader both of which support this behaviour (though the markdown reader in neovim is limited to the selected buffer, so doesn't quite work)

(mentioned in the Discord)

[skatkov]

What would be the desired behavior in case it's not a relative filepath? It could be an URL

[bashbunni]

@skatkov yeah still clickable :)