Add preferSwiftStringAPI rule (#2292)
Replaces the Objective-C bridged `replacingOccurrences(of:with:)` with
Swift-native `replacing(_:with:)`. The ObjC method has known Unicode
bugs (e.g. corrupting emoji flag sequences); the Swift equivalent is
safer, shorter, and available from Swift 5.7+.
Only the two-argument form is transformed — calls with `options:` or
`range:` are left unchanged as they have no direct Swift equivalent.