Figure out how to export `alt=variant` versions of elements

Many of the elements in CLDR have alt=variant versions that are supposed to be used "in some circumstances".

Figure out how we should export these.

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Examples

eraNames

common/main/zu.xml

<eraNames>
  <era type="0">BC</era>
  <era type="0" alt="variant">BCE</era>
  <era type="1">AD</era>
  <era type="1" alt="variant">CE</era>
</eraNames>

Currency symbol

common/main/af.xml

<currency type="TRY">
  <displayName>Turkse lira</displayName>
  <displayName count="one">Turkse lira</displayName>
  <displayName count="other">Turkse lira</displayName>
  <symbol draft="contributed">TRY</symbol>
  <symbol alt="narrow" draft="contributed">₺</symbol>
  <symbol alt="variant" draft="contributed">TL</symbol>
</currency>

territory names

common/main/de.xml

<territory type="CD">Kongo-Kinshasa</territory>
<territory type="CD" alt="variant">Kongo (Demokratische Republik)</territory>

common/main/de.xml

language names

<language type="ckb">Zentralkurdisch</language>
<language type="ckb" alt="menu">Kurdisch (Sorani)</language>
<language type="ckb" alt="variant">↑↑↑</language>

dayPeriod

common/main/en.xml

<dayPeriod type="am">AM</dayPeriod>
<dayPeriod type="am" alt="variant">am</dayPeriod>

common/main/en.xml

<field type="dayperiod-short">
  <displayName>AM/PM</displayName>
  <displayName alt="variant">am/pm</displayName>
</field>

script names

common/main/en.xml

<script type="Arab">Arabic</script>
<script type="Arab" alt="variant">Perso-Arabic</script>

Start by examining the exporter behavior against the cited CLDR examples in common/main/zu.xml, common/main/af.xml, common/main/de.xml, and common/main/en.xml. Determine the export policy for alt="variant" across the listed element types, and consider how that policy should be verified before treating the work as complete.