[w3c/payment-method-basic-card] Standardize `environment` member (#41)

Edge currently extends Basic Card with an `environment` member... it's currently used for testing in Microsoft Wallet, but other implementers would like to also support it for the same reason. 

To standardize it, we need to: 
 * [ ] Get assurance from Microsoft that it won't change (or will change, if we ask for changes... like adding a few more addresses). 
 * [ ] Reverse engineer the data we can actually get back from it.
 * [ ] Spec it up. 
 * [ ] Write tests 

cc @molly26dalton 

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https://github.com/w3c/payment-method-basic-card/issues/41