feat: add currency property and option

[ramsey]

Description

Adds the ability to set the currency type for the locale.

Motivation and context

This feature is not defined in ECMA-402, but it is inspired by the Intl Info Proposal and is implemented to behave like the other properties.

How has this been tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

PR checklist

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING.md document.
• I have added tests to cover my changes.

[codecov]

Codecov Report

Merging #12 (46f9e40) into main (23fd04e) will increase coverage by 0.24%.
The diff coverage is 89.13%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #12      +/-   ##
==========================================
+ Coverage   82.27%   82.52%   +0.24%     
==========================================
  Files          12       12              
  Lines         942      984      +42     
  Branches      182      190       +8     
==========================================
+ Hits          775      812      +37     
- Misses         94       97       +3     
- Partials       73       75       +2     

Flag	Coverage Δ
unittests	82.52% <89.13%> (+0.24%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
src/ecma402/locale.cpp	77.14% <87.80%> (+0.90%)	⬆️
src/ecma402/language_tag.cpp	80.68% <100.00%> (+0.42%)	⬆️