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/Cargo.toml

@@ -51,7 +51,6 @@ lsp.workspace = true
 menu.workspace = true
 multi_buffer.workspace = true
 open_ai = { workspace = true, features = ["schemars"] }
-ordered-float.workspace = true
 parking_lot.workspace = true
 paths.workspace = true
 picker.workspace = true
@@ -71,6 +70,7 @@ settings.workspace = true
 similar.workspace = true
 smallvec.workspace = true
 smol.workspace = true
+streaming_diff.workspace = true
 strum.workspace = true
 telemetry.workspace = true
 telemetry_events.workspace = true