curate format-dates: support date ranges

### Context NCBI Datasets has started returning date ranges in the format of `[2001 TO 2002]`, which has caused errors in the ingest workflow during `augur curate format-dates` ([norovirus](https://github.com/nextstrain/norovirus/actions/runs/26050724373/attempts/1), [rabies](https://github.com/nextstrain/rabies/actions/runs/26050822788/attempts/1), [seasonal-cov](https://github.com/nextstrain/seasonal-cov/actions/runs/26051659176/attempts/1)). Related to https://github.com/nextstrain/augur/issues/1494, but different in that this needs to support parsing an _existing_ date range format rather than _apply_ a date range to a field. ### Description `curate format-dates` should be able to convert the date range to the Augur supported date range format `YYYY-MM-DD/YYYY-MM-DD`. `[2001 TO 2002]` -> ` 2001-XX-XX/2002-XX-XX`