Support geopoint type for Typesense 0.21

[kishorenc]

Thank you very much @happy-san for the amazing work on the Dart client.

Is your feature request related to a problem? Please describe.

Typesense 0.21 introduces geo search so you can index and search on lat/long fields. A new type geopoint is introduced for this purpose.

Please see usage here: https://gist.github.com/kishorenc/1f537c728f89960048272a03d36bad01

Describe the solution you'd like

• Update Type enum in Field class. #91

• Update exportJSONL function in Documents class. #92

[happy-san]

Thank you for the kind words.

I've added this to version 0.1.1 milestone.