Increase metafield support beyond 250

[freakdesign]

Some stores using Langify are finding that they are quickly hitting 250+ metafields at shop level. This means that the "custom field" setup metafields might not be found (if they fall outside of that 250 group) so the tool won't shown any.

Custom Fields has never bothered to grab any more than 250 (which was deemed to be a reasonable limit), so an update is needed to get more.