PatternItem currently just holds an array of Objects, the first of which is a string identifier for its type. We should convert this to structured data, such as a type enum and an optional length field.

See https://github.com/flutter/packages/pull/7631/files/a85acb23fddcf2b3abe6a8dc8a5f63a40958b8ba#r1757488052