Introduce diff crate to unite BufferDiff and BufferChangeSet (#24392)

This is a refactoring PR that does three things:

- First, it introduces a new `diff` crate that holds the previous
contents of the `git::diff` module, plus the `BufferChangeSet` type
formerly of `project::buffer_store`. The new crate is necessary since
simply moving `BufferChangeSet` into `git::diff` results in a dependency
cycle due to the use of `language::Buffer` to represent the diff base in
`BufferChangeSet`.
- Second, it renames the two main types in the new diff crate:
`BufferDiff` becomes `BufferDiffSnapshot`, and `BufferChangeSet` becomes
`BufferDiff`. This reflects that the relationship between these two
types (immutable cheaply-cloneable "value" type + stateful "resource
type" with subscriptions) mirrors existing pairs like
`Buffer`/`BufferSnapshot`. References to "change sets" throughout the
codebase are updated to refer to "diffs" instead.
- Finally, it moves the base_text field of the new BufferDiff type to
BufferDiffSnapshot.

Release Notes:

- N/A

---------

Co-authored-by: maxbrunsfeld <max@zed.dev>

crates/proto/src/proto.rs

@@ -219,10 +219,10 @@ messages!(
     (GetImplementationResponse, Background),
     (GetLlmToken, Background),
     (GetLlmTokenResponse, Background),
-    (OpenUnstagedChanges, Foreground),
-    (OpenUnstagedChangesResponse, Foreground),
-    (OpenUncommittedChanges, Foreground),
-    (OpenUncommittedChangesResponse, Foreground),
+    (OpenUnstagedDiff, Foreground),
+    (OpenUnstagedDiffResponse, Foreground),
+    (OpenUncommittedDiff, Foreground),
+    (OpenUncommittedDiffResponse, Foreground),
     (GetUsers, Foreground),
     (Hello, Foreground),
     (IncomingCall, Foreground),
@@ -424,8 +424,8 @@ request_messages!(
     (GetProjectSymbols, GetProjectSymbolsResponse),
     (GetReferences, GetReferencesResponse),
     (GetSignatureHelp, GetSignatureHelpResponse),
-    (OpenUnstagedChanges, OpenUnstagedChangesResponse),
-    (OpenUncommittedChanges, OpenUncommittedChangesResponse),
+    (OpenUnstagedDiff, OpenUnstagedDiffResponse),
+    (OpenUncommittedDiff, OpenUncommittedDiffResponse),
     (GetSupermavenApiKey, GetSupermavenApiKeyResponse),
     (GetTypeDefinition, GetTypeDefinitionResponse),
     (LinkedEditingRange, LinkedEditingRangeResponse),
@@ -546,8 +546,8 @@ entity_messages!(
     GetProjectSymbols,
     GetReferences,
     GetSignatureHelp,
-    OpenUnstagedChanges,
-    OpenUncommittedChanges,
+    OpenUnstagedDiff,
+    OpenUncommittedDiff,
     GetTypeDefinition,
     InlayHints,
     JoinProject,