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/assistant/src/codegen.rs

@@ -7,6 +7,7 @@ use editor::{Anchor, MultiBuffer, MultiBufferSnapshot, ToOffset, ToPoint};
 use futures::{channel::mpsc, SinkExt, Stream, StreamExt};
 use gpui::{EventEmitter, Model, ModelContext, Task};
 use language::{Rope, TransactionId};
+use multi_buffer::MultiBufferRow;
 use std::{cmp, future, ops::Range};
 
 pub enum Event {
@@ -100,7 +101,7 @@ impl Codegen {
             .suggested_indents(selection_start.row..selection_start.row + 1, cx)
             .into_values()
             .next()
-            .unwrap_or_else(|| snapshot.indent_size_for_line(selection_start.row));
+            .unwrap_or_else(|| snapshot.indent_size_for_line(MultiBufferRow(selection_start.row)));
 
         let response = CompletionProvider::global(cx).complete(prompt);
         self.generation = cx.spawn(|this, mut cx| {