Align swift-format configuration with other projects (#150)

* Align `swift-format` configuration with other projects

No code change. We already followed these rules.

* UseSynthesizedInitializer still needs to be set to false

The fix for swiftlang/swift-format#591 is not in `509.0.0` but only in `main`: swiftlang/swift-format@b896e0f

.swift-format

@@ -29,7 +29,7 @@
     "NeverForceUnwrap" : false,
     "NeverUseForceTry" : false,
     "NeverUseImplicitlyUnwrappedOptionals" : false,
-    "NoAccessLevelOnExtensionDeclaration" : false,
+    "NoAccessLevelOnExtensionDeclaration" : true,
     "NoAssignmentInExpressions" : true,
     "NoBlockComments" : true,
     "NoCasesWithOnlyFallthrough" : true,
@@ -53,6 +53,6 @@
     "ValidateDocumentationComments" : false
   },
   "spacesAroundRangeFormationOperators" : false,
-  "tabWidth" : 8,
+  "tabWidth" : 4,
   "version" : 1
 }