Add an editor: diff clipboard with selection action (#33283)

https://github.com/user-attachments/assets/d472fbdd-7736-4bd7-8a90-8cca356b2815

This PR adds `editor: diff clipboard with selection` - good for spotting
the differences in eerily-similar code, which is when refactoring code,
as you need to see what needs to be passed in in order to maintain
previous behavior of both snippets.

1. Copy some text from anywhere
2. Highlight some text in Zed
3. Run `editor: diff clipboard with selection`

Like JetBrains' IDEs and VS Code with the `PartialDiff` package, if the
selection is empty, we take the entire buffer as the selection.

Caveats:

- We do not know the language of the text in the clipboard. I went ahead
and just assumed that in most cases, it will be the same language as the
selected text, which does mean we will highlight the old text
incorrectly if they are copying from a different language, but I think
in most cases, it will be the same, and the alternative of always having
no syntax highlighting is worse. PyCharm seems to do the same thing.

Release Notes:

- Added an `editor: diff clipboard with selection` action

---------

Co-authored-by: Junkui Zhang <364772080@qq.com>
Co-authored-by: Ben Kunkle <ben@zed.dev>

crates/gpui/src/app/entity_map.rs

@@ -370,7 +370,7 @@ impl std::fmt::Debug for AnyEntity {
     }
 }
 
-/// A strong, well typed reference to a struct which is managed
+/// A strong, well-typed reference to a struct which is managed
 /// by GPUI
 #[derive(Deref, DerefMut)]
 pub struct Entity<T> {