Era labels sometimes come before values

There should be an attribute of era that specifies whether the label is conventionally placed before or after the value, e.g. "AD 1" vs. "1 BC".

Start by locating how era labels and values are represented and formatted in the R package, then inspect existing tests for era formatting. Define the placement information so cases like "AD 1" and "1 BC" can be distinguished, and add coverage demonstrating both conventions.

Written by the indexing model from the issue text.