Add ISO code for Zimbabwe Dollar

[saebs]

Support for Zimbabwe Currency according to ISO4217 standard.

[varunsrin]

Thanks @saebs.

I had originally left this out, since ZWL had been demonetized before the library was written. Out of curiosity, is it still useful to have currencies like this in the lib?

[saebs]

Okay i understand where you are coming from. Ours is a confusing case. However the point of correction is that, we have an existing "quasy-currency" if i should say so myself. It is called the RTGS Dollar It uses the ZWL code in the banking systems. to quote a local tech blog "Despite demonetisation & the new RTGS dollar in 2019, the ISO currency code has not changed from the one used for the 4th dollar after the last demonetisation in 2009. The introduction of the RTGS dollar has not caused a reassignment of the ISO currency code. It’s still ZWL."
So there i compelling case to include it locally. I am developing a payments library and chose this lib cause of its simplicity, quite intuitive.

thank you for responding @varunsrin

for further reference: https://www.techzim.co.zw/2019/03/now-we-know-why-the-rtgs-is-referred-to-as-zwl-thanks-alex-magaisa/

[varunsrin]

Thank you for the context, this makes sense and I did not realize the ISO code was still in use. I think it makes sense to merge this in.