Add streaming_diff crate (#23264)

This PR extracts the streaming diff implementation to its own
`streaming_diff` crate.

It was duplicated between `assistant` and `assistant2`, but their
implementations were exactly the same (and I don't see a reason why they
would need to diverge).

Release Notes:

- N/A

crates/assistant/src/inline_assistant.rs

@@ -1,7 +1,6 @@
 use crate::{
-    humanize_token_count, AssistantPanel, AssistantPanelEvent, CharOperation,
-    CycleNextInlineAssist, CyclePreviousInlineAssist, LineDiff, LineOperation, RequestType,
-    StreamingDiff,
+    humanize_token_count, AssistantPanel, AssistantPanelEvent, CycleNextInlineAssist,
+    CyclePreviousInlineAssist, RequestType,
 };
 use anyhow::{anyhow, Context as _, Result};
 use assistant_settings::AssistantSettings;
@@ -56,6 +55,7 @@ use std::{
     task::{self, Poll},
     time::{Duration, Instant},
 };
+use streaming_diff::{CharOperation, LineDiff, LineOperation, StreamingDiff};
 use telemetry_events::{AssistantEvent, AssistantKind, AssistantPhase};
 use terminal_view::terminal_panel::TerminalPanel;
 use text::{OffsetRangeExt, ToPoint as _};