feat(isVAT): improved ABN (AU GST/VAT ID) validation

[matthewberryman]

Implemented correct ABN (AU GST/VAT ID) validation per the official reference (see also this reference and implementation)

Checklist

• PR contains only changes related; no stray files, etc.
• README updated: N/A
• Tests written (where applicable)
• References provided in PR (where applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b958bd7) 99.95% compared to head (730fbcc) 99.95%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2343   +/-   ##
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  Coverage   99.95%   99.95%           
=======================================
  Files         107      107           
  Lines        2454     2459    +5     
  Branches      619      621    +2     
=======================================
+ Hits         2453     2458    +5     
  Partials        1        1           

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[profnandaa]

LGTM, thanks for your contrib!