tantivy new: Choose Field Type could support case insensitive input

Nitpick, but this would make the interactive cli slightly easier to use

```
New field name  ? myfield
Choose Field Type (Text/u64/i64/f64/Date/Facet/Bytes/Json/bool/IpAddr) ? text
Error: Invalid input. Options are (Text/u64/i64/f64/Date/Facet/Bytes/Json/bool/IpAddr)
Choose Field Type (Text/u64/i64/f64/Date/Facet/Bytes/Json/bool/IpAddr) ? Text
Should the field be stored (Y/N) ? y
```