New invoice line codes for EN16931 compatibility

[samlown]

• bill.Line has new properties: identifier, period, order, and code, in order to comply with EN16931 requirements.

Pre-Review Checklist

• I have performed a self-review of my code.
• I have added thorough tests with at least 90% code coverage.
• I've modified or created example GOBL documents to show my changes in use, if appropriate.
• When adding or modifying a tax regime or addon, I've added links to the source of the changes, either structured or in the comments.
• I've run go generate . to ensure that the Schemas and Regime data are up to date.
• All linter warnings have been reviewed and fixed.
• I've been obsessive with pointer nil checks to avoid panics.
• The CHANGELOG.md has been updated with an overview of my changes.
• Requested a review from @samlown.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.67%. Comparing base (9f5e454) to head (77626ca).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #459      +/-   ##
==========================================
+ Coverage   85.15%   85.67%   +0.52%     
==========================================
  Files         255      255              
  Lines       12576    12582       +6     
==========================================
+ Hits        10709    10780      +71     
+ Misses       1518     1454      -64     
+ Partials      349      348       -1     

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[cavalle]

+100