Introduce DisplayRow, MultiBufferRow newtypes and BufferRow type alias (#11656)

Part of https://github.com/zed-industries/zed/issues/8081

To avoid confusion and bugs when converting between various row `u32`'s,
use different types for each.
Further PRs should split `Point` into buffer and multi buffer variants
and make the code more readable.

Release Notes:

- N/A

---------

Co-authored-by: Piotr <piotr@zed.dev>

crates/outline/src/outline.rs

@@ -486,6 +486,7 @@ mod tests {
             editor
                 .highlighted_display_rows(HashSet::default(), cx)
                 .into_keys()
+                .map(|r| r.0)
                 .collect()
         })
     }