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-protocol/src/ty.rs

@@ -135,7 +135,7 @@ impl Type {
             Type::Closure => String::from("closure"),
             Type::Bool => String::from("bool"),
             Type::CellPath => String::from("cell-path"),
-            Type::Date => String::from("date"),
+            Type::Date => String::from("datetime"),
             Type::Duration => String::from("duration"),
             Type::Filesize => String::from("filesize"),
             Type::Float => String::from("float"),
@@ -162,7 +162,7 @@ impl Display for Type {
             Type::Closure => write!(f, "closure"),
             Type::Bool => write!(f, "bool"),
             Type::CellPath => write!(f, "cell-path"),
-            Type::Date => write!(f, "date"),
+            Type::Date => write!(f, "datetime"),
             Type::Duration => write!(f, "duration"),
             Type::Filesize => write!(f, "filesize"),
             Type::Float => write!(f, "float"),

crates/nu-protocol/src/value/mod.rs

@@ -329,7 +329,7 @@ impl Value {
         if let Value::Date { val, .. } = self {
             Ok(*val)
         } else {
-            self.cant_convert_to("date")
+            self.cant_convert_to("datetime")
         }
     }