assistant2: Add create buffer tool (#11219)

This PR adds a new tool to the `assistant2` crate that allows the
assistant to create a new buffer with some content.

Release Notes:

- N/A

---------

Co-authored-by: Nathan <nathan@zed.dev>

crates/assistant2/src/tools.rs

@@ -1,3 +1,5 @@
+mod create_buffer;
 mod project_index;
 
+pub use create_buffer::*;
 pub use project_index::*;