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-cmd-lang/src/core_commands/describe.rs

@@ -103,7 +103,7 @@ impl Command for Describe {
                                 "category" => Value::test_string("default"),
                             )),
                         )),
-                        "first_commit" => Value::test_string("date"),
+                        "first_commit" => Value::test_string("datetime"),
                         "my_duration" => Value::test_string("duration"),
                     )),
                 ))),