[w3c/payment-method-id] Better define validity of standardized PMIs? (#48)

As per spec, the following PMIs are all valid:

```JS
    "0",
    "021213213",
    "-valid-",
    "-valid-123",
    "--------",
```

I think what we want is:

```ABNF
stdpmi = part *("-" part)
part = loweralpha *(DIGIT / loweralpha)
loweralpha =  %x61-7A
```


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