Secular/neutral eras

Currently for gregorian we have the following in "root":

<eras>

<eraNames>

<alias source="locale" path="../eraAbbr"/>

</eraNames>

<eraAbbr>

<era type="0">BE</era>

<!-- = 544 BC gregorian. -->

</eraAbbr>

<eraNarrow>

<alias source="locale" path="../eraAbbr"/>

</eraNarrow>

</eras>

and we have the following in "en", for example:

<eras>

<eraNames>

<era type="0">Before Christ</era>

<era type="1">Anno Domini</era>

</eraNames>

<eraAbbr>

<era type="0">BC</era>

<era type="1">AD</era>

</eraAbbr>

<eraNarrow>

<era type="0">B</era>

<era type="1">A</era>

</eraNarrow>

</eras>

We need a way that locales can provide data for an alternate secular/neutral era name if their default era name has a religious basis (as with the current en/gregorian/era naming).

Since many locales already use a secular/neutral name as the default for gregorian, this would need to fall back to the default name in the same locale.

The easiest way to do this is just provide <eras alt="variant">l in locales that need it. A request for the "variant" form will fall back to the default form in the same locale if no "variant" form is present. So in "en" we would add this:

<eras alt="variant">

<eraNames>

<era type="0">Before Common Era</era>

<era type="1">Common Era</era>

</eraNames>

<eraAbbr>

<era type="0">BCE</era>

<era type="1">CE</era>

</eraAbbr>

</eras>

(We could use other names for the alt form such as "secular" or "neutral" but "variant" is more general and already widely supported.)