chore(apple): add .swift-format file (#9566)

When using alternative editors other than Xcode, Intellisense is usually
provided by `sourcekit-lsp`. The LSP server also uses `swift-format`
under the hood to format the code but appears to default to using 4
spaces for indentation. In order to standardise on how much indentation
is used, we add a `.swift-format` file that specifies is.

swift/apple/.swift-format

@@ -0,0 +1,6 @@
+{
+    "version": 1,
+    "indentation": {
+        "spaces": 2
+    }
+}