Unifying language for easier localization

[stuartpb]

Some rules in places like username.value.must, like must begin with a letter, right now I'm copying whatever the stated language on the site is - so if the site spells "begin with" as "start with", I do that, too.

To make it easier to localize common rules, though, I should maybe start standardizing on some unified terminology / patterns for "freeform" field like this, so a consumer could ship with a rule that localizes "start with a letter" into its supported languages, even if the profile itself doesn't include "start with a letter" translated into that language. (Indeed, there could even be a policy that certain common strings don't get translated in profiles, under this mechanic.)

[stuartpb]

The specific "begin with X" pattern is being encoded in #251, so that's not a great example.

Indeed, that definitely points to a shortcoming of this: if a pattern is so common that there needs to be standard language to describe it, it's become straightforward enough to structure in fields.

That said, there's still #138, and #137, and the question of representing patterns in the interim before the schema is structured to represent them, so this issue isn't completely invalid - but it's definitely one to be wary of going too deep into.

[stuartpb]

Further discussion should continue into opws/opws-guidelines#6.