Rename user-facing 'date' to 'datetime' (#15264)

We only have one valid `datetime` type, but the string representation of
that type was `date`. This PR updates the string representation of the
`datetime` type to be `datetime` and updates other affected
dependencies:

* A `describe` example that used `date`
* The style computer automatically recognized the new change, but also
changed the default `date: purple` to `datetime: purple`.
* Likewise, changed the `default_config.nu` to populate
`$env.config.color_config.datetime`
* Likewise, the dark and light themes in `std/config`
* Updates tests
* Unrelated, but changed the `into value` error messages to use
*"datetime"* if there's an issue.

Fixes #9916 and perhaps others.

## Breaking Changes:

* Code that expected `describe` to return a `date` will now return a
`datetime`
* User configs and themes that override `$env.config.color_config.date`
will need to be updated to use `datetime`

crates/nu-command/tests/format_conversions/nuon.rs

@@ -303,7 +303,7 @@ fn from_nuon_datetime() {
         "#
     ));
 
-    assert_eq!(actual.out, "date");
+    assert_eq!(actual.out, "datetime");
 }
 
 #[test]