Copy/paste images into editors (Mac only) (#15782)

For future reference: WIP branch of copy/pasting a mixture of images and
text: https://github.com/zed-industries/zed/tree/copy-paste-images -
we'll come back to that one after landing this one.

Release Notes:

- You can now paste images into the Assistant Panel to include them as
context. Currently works only on Mac, and with Anthropic models. Future
support is planned for more models, operating systems, and image
clipboard operations.

---------

Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Mikayla <mikayla@zed.dev>
Co-authored-by: Jason <jason@zed.dev>
Co-authored-by: Kyle <kylek@zed.dev>

crates/paths/src/paths.rs

@@ -170,6 +170,12 @@ pub fn contexts_dir() -> &'static PathBuf {
     })
 }
 
+/// Returns the path within the contexts directory where images from contexts are stored.
+pub fn context_images_dir() -> &'static PathBuf {
+    static CONTEXT_IMAGES_DIR: OnceLock<PathBuf> = OnceLock::new();
+    CONTEXT_IMAGES_DIR.get_or_init(|| contexts_dir().join("images"))
+}
+
 /// Returns the path to the contexts directory.
 ///
 /// This is where the prompts for use with the Assistant are stored.