Added support for postgres citext

[gracepetryk]

My company uses citext to store a bunch of case insensitive columns, this change should mark in any columns in the annotation with that type as Strings.

[gracepetryk]

docs for the type if you're not familiar: https://www.postgresql.org/docs/current/citext.html

[alisnic]

thanks for PR! can also please add an example and assertion to https://github.com/iftheshoefritz/solargraph-rails/blob/main/spec/solargraph-rails/schema_spec.rb?

[gracepetryk]

done, lmk if there's anything else you need me to do

[alisnic]

Awesome! thank you 🙏