Cast character to era_yr?

It could be possible to cast characters to era_yrs (i.e. the inverse of #11) in some circumstances. E.g. `"1 BCE"` could be interpreted as `yr(1, "BCE")`. But maybe this is too magical?

Start by reading issue #11 and the existing character-casting behavior in the package. Determine which character forms should be accepted as era_yr values and whether the proposed conversion is sufficiently predictable. Done means the supported scope is agreed and covered by tests, but this issue names no files or test entry points.

Written by the indexing model from the issue text.