feat(citext): support postgres citext (#2478)

* feat(citext): implement citext for postgres

* feat(citext): add citext -> String conversion test

* feat(citext): fix ltree -> citree

* feat(citext): add citext to the setup.sql

* chore: address nits to #2478

* Rename `PgCitext` to `PgCiText`
* Document when use of `PgCiText` is warranted
* Document potentially surprising `PartialEq` behavior
* Test that the macros consider `CITEXT` to be compatible with `String` and friends

* doc: add `PgCiText` to `postgres::types` listing

* chore: restore missing trailing line break to `tests/postgres/setup.sql`

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Co-authored-by: Austin Bonander <austin@launchbadge.com>