Accept MIME types as input and resolve to UTI strings

[sfriedenberg-etsy]

Allow users to pass MIME types (e.g. text/html, application/pdf, image/png) instead of UTI strings. Tacky would resolve them to the corresponding UTI before interacting with the pasteboard.

macOS provides UTType(mimeType:) for this conversion. In Rust, this could be done via the UTTypeCreatePreferredIdentifierForTag Core Services function or a built-in lookup table for common types.