Support case insensitive string enum values

[k-paxian]

Instead of doing that:

@jsonSerializable
enum RecordType { asset, series }

final customAdapter = JsonMapperAdapter(
    enumValues: {
      RecordType: EnumDescriptor(
          values: RecordType.values,
          mapping: <RecordType, String>{
            RecordType.asset: 'Asset',
            RecordType.series: 'Series'
          })
    });

should be like this:

@jsonSerializable
enum RecordType { asset, series }

final customAdapter = JsonMapperAdapter(
    enumValues: {
      RecordType: EnumDescriptor(
          values: RecordType.values,
          caseInsensitive: true
    });

Inspired by this