layout: Split LineItem layout into a new file (#31039)

This is just a bit of code movement that trims down the size of the
`inline.rs` file in order to make it a bit more manageable. It leads the
way to more refactoring and cleanup in the future.

components/layout_2020/flow/mod.rs

@@ -36,6 +36,7 @@ use crate::ContainingBlock;
 mod construct;
 pub mod float;
 pub mod inline;
+mod line;
 mod root;
 
 pub(crate) use construct::BlockContainerBuilder;