refactor: Simplify element parsing by introducing parseFromParts method

[hangy]

Follow-up to #14, as I don't know if you want this kind of change.

In #14, we need to combine AI data and value, so that they can be passed to the existing parsers.

gs1_barcode_parser/lib/src/barcode_parser.dart

Line 225 in 3460633

final aiData = '$aiCode$value';

This is a tiny bit inefficient, because this newly allocated string is immediately split by the parser. This PR introduces a parseFromParts methods to the parsers, which helps avoid these unnecessary allocations.

Not sure if the optimization is worth the updated API interface, though. No bad feelings if this PR is declined. 😊

[sachebotarev]

Hi,
Thank you for your pull request. I’ll review the changes during the week and get back to you with comments :).