feat!(prefer-readonly-type): turn option "allowMutableReturnType" on by default

BREAKING CHANGE: allowMutableReturnType is now on by default

re #153

Author: RebeccaStevens

docs/rules/prefer-readonly-type.md

@@ -106,7 +106,7 @@ The default options:
 ```ts
 {
   allowLocalMutation: false,
-  allowMutableReturnType: false,
+  allowMutableReturnType: true,
   checkImplicit: false,
   ignoreClass: false,
   ignoreInterface: false,

src/rules/prefer-readonly-type.ts

@@ -75,7 +75,7 @@ const defaultOptions: Options = {
   ignoreInterface: false,
   ignoreCollections: false,
   allowLocalMutation: false,
-  allowMutableReturnType: false,
+  allowMutableReturnType: true,
 };
 
 // The possible error messages.