[xi-core-lib] Missing panic conditon on docs

Style::new() has assert statement, but panic condition is missing on rust docs. Better to add panic condition like other crates' api

https://github.com/xi-editor/xi-editor/blob/a2dea3059312795c77caadc639df49bf8a7008eb/rust/core-lib/src/styles.rs#L80-L102