[Swift] Moves capacity outside of Storage (#8650)

- Cleans up capacity usage within the lib and moves it outside of the Storage

-  Use overflow operators

swift/Sources/FlexBuffers/Reader/FlexBufferVector.swift

@@ -25,7 +25,7 @@ extension FlexBufferVector {
     json += "["
     for i in 0..<count {
       if let val = self[i]?.jsonString() {
-        let comma = i == count - 1 ? "" : ", "
+        let comma = i == count &- 1 ? "" : ", "
         json += "\(val)\(comma)"
       }
     }

swift/Sources/FlexBuffers/Reader/Map.swift

@@ -52,7 +52,7 @@ extension Map {
     json += "{"
     for i in 0..<count {
       if let key = keys[i]?.cString {
-        let comma = i == count - 1 ? "" : ", "
+        let comma = i == count &- 1 ? "" : ", "
         let value = values[i]?.jsonString() ?? StaticJSON.null
         json += "\"\(key)\": \(value)\(comma)"
       }