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/collab/src/rpc.rs

@@ -309,8 +309,8 @@ impl Server {
             .add_request_handler(forward_read_only_project_request::<proto::ResolveInlayHint>)
             .add_request_handler(forward_read_only_project_request::<proto::OpenBufferByPath>)
             .add_request_handler(forward_read_only_project_request::<proto::GitBranches>)
-            .add_request_handler(forward_read_only_project_request::<proto::OpenUnstagedChanges>)
-            .add_request_handler(forward_read_only_project_request::<proto::OpenUncommittedChanges>)
+            .add_request_handler(forward_read_only_project_request::<proto::OpenUnstagedDiff>)
+            .add_request_handler(forward_read_only_project_request::<proto::OpenUncommittedDiff>)
             .add_request_handler(
                 forward_mutating_project_request::<proto::RegisterBufferWithLanguageServers>,
             )