Extract multi_buffer module out of editor (#3170)

Release Notes:

- N/A

crates/assistant/src/assistant_panel.rs

@@ -296,7 +296,7 @@ impl AssistantPanel {
         };
 
         let selection = editor.read(cx).selections.newest_anchor().clone();
-        if selection.start.excerpt_id() != selection.end.excerpt_id() {
+        if selection.start.excerpt_id != selection.end.excerpt_id {
             return;
         }
         let snapshot = editor.read(cx).buffer().read(cx).snapshot(cx);

crates/assistant/src/codegen.rs

@@ -1,10 +1,11 @@
 use crate::streaming_diff::{Hunk, StreamingDiff};
 use ai::completion::{CompletionProvider, OpenAIRequest};
 use anyhow::Result;
-use editor::{multi_buffer, Anchor, MultiBuffer, MultiBufferSnapshot, ToOffset, ToPoint};
+use editor::{Anchor, MultiBuffer, MultiBufferSnapshot, ToOffset, ToPoint};
 use futures::{channel::mpsc, SinkExt, Stream, StreamExt};
 use gpui::{Entity, ModelContext, ModelHandle, Task};
 use language::{Rope, TransactionId};
+use multi_buffer;
 use std::{cmp, future, ops::Range, sync::Arc};
 
 pub enum Event {