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>

Cargo.toml

@@ -32,6 +32,7 @@ members = [
     "crates/db",
     "crates/deepseek",
     "crates/diagnostics",
+    "crates/diff",
     "crates/docs_preprocessor",
     "crates/editor",
     "crates/evals",
@@ -231,6 +232,7 @@ copilot = { path = "crates/copilot" }
 db = { path = "crates/db" }
 deepseek = { path = "crates/deepseek" }
 diagnostics = { path = "crates/diagnostics" }
+diff = { path = "crates/diff" }
 editor = { path = "crates/editor" }
 extension = { path = "crates/extension" }
 extension_host = { path = "crates/extension_host" }