Australian Regime

[FuzzyGrim]

Adds GST support for Australia with 10% standard rate and common zero-rated exemptions.

Pre-Review Checklist

• I've read the CONTRIBUTING.md guide.
• 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

❌ Patch coverage is 90.83969% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.87%. Comparing base (5c1711c) to head (9a8c9f8).

Files with missing lines	Patch %	Lines
regimes/au/invoices.go	80.32%	8 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #613      +/-   ##
==========================================
- Coverage   90.87%   90.87%   -0.01%     
==========================================
  Files         292      295       +3     
  Lines       16503    16634     +131     
==========================================
+ Hits        14997    15116     +119     
- Misses       1195     1203       +8     
- Partials      311      315       +4     

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