Refactor: moving to single cbc.Definition structure

[samlown]

Significant refactor moving from the cbc.KeyDefinition and cbc.ValueDefinition back to a single cbc.Definition that covers both key and code definitions.

Since the cbc.Code was recently made a lot more flexible, the use of "values" and tax.ExtValue no longer makes much sense.

Checklist before requesting a review

• 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 the 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 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

Attention: Patch coverage is 98.51852% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.89%. Comparing base (eb70ac6) to head (98f196f).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
cbc/notes.go	0.00%	1 Missing ⚠️
tax/regime_def.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #441      +/-   ##
==========================================
+ Coverage   82.28%   82.89%   +0.61%     
==========================================
  Files         242      241       -1     
  Lines       11881    11892      +11     
==========================================
+ Hits         9776     9858      +82     
+ Misses       1737     1673      -64     
+ Partials      368      361       -7     

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