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/streaming_diff/Cargo.toml

@@ -0,0 +1,20 @@
+[package]
+name = "streaming_diff"
+version = "0.1.0"
+edition = "2021"
+publish = false
+license = "GPL-3.0-or-later"
+
+[lints]
+workspace = true
+
+[lib]
+path = "src/streaming_diff.rs"
+
+[dependencies]
+ordered-float.workspace = true
+rope.workspace = true
+
+[dev-dependencies]
+rand.workspace = true
+util = { workspace = true, features = ["test-support"] }

crates/streaming_diff/LICENSE-GPL

@@ -0,0 +1 @@
+../../LICENSE-GPL