Make custom value type handling more consistent (#12230)

[Context on
Discord](https://discord.com/channels/601130461678272522/855947301380947968/1219425984990806207)

# Description

- Rename `CustomValue::value_string()` to `type_name()` to reflect its
usage better.
- Change print behavior to always call `to_base_value()` first, to give
the custom value better control over the output.
- Change `describe --detailed` to show the type name as the subtype,
rather than trying to describe the base value.
- Change custom `Type` to use `type_name()` rather than `typetag_name()`
to make things like `PluginCustomValue` more transparent

One question: should `describe --detailed` still include a description
of the base value somewhere? I'm torn on it, it seems possibly useful
for some things (maybe sqlite databases?), but having `describe -d` not
include the custom type name anywhere felt weird. Another option would
be to add another method to `CustomValue` for info to be displayed in
`describe`, so that it can be more type-specific?

# User-Facing Changes
Everything above has implications for printing and `describe` on custom
values

# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

crates/nu-plugin/src/plugin/interface/engine/tests.rs

@@ -1138,7 +1138,7 @@ impl CustomValue for CantSerialize {
         Value::custom_value(Box::new(self.clone()), span)
     }
 
-    fn value_string(&self) -> String {
+    fn type_name(&self) -> String {
         "CantSerialize".into()
     }
 

crates/nu-plugin/src/protocol/plugin_custom_value.rs

@@ -55,7 +55,7 @@ impl CustomValue for PluginCustomValue {
         Value::custom_value(Box::new(self.clone()), span)
     }
 
-    fn value_string(&self) -> String {
+    fn type_name(&self) -> String {
         self.name().to_owned()
     }
 
@@ -212,7 +212,7 @@ impl PluginCustomValue {
         custom_value: &dyn CustomValue,
         span: Span,
     ) -> Result<PluginCustomValue, ShellError> {
-        let name = custom_value.value_string();
+        let name = custom_value.type_name();
         let notify_on_drop = custom_value.notify_plugin_on_drop();
         bincode::serialize(custom_value)
             .map(|data| PluginCustomValue::new(name, data, notify_on_drop, None))
@@ -297,7 +297,7 @@ impl PluginCustomValue {
                     } else {
                         // Only PluginCustomValues can be sent
                         Err(ShellError::CustomValueIncorrectForPlugin {
-                            name: val.value_string(),
+                            name: val.type_name(),
                             span,
                             dest_plugin: source.name().to_owned(),
                             src_plugin: None,

crates/nu-plugin/src/protocol/plugin_custom_value/tests.rs

@@ -19,7 +19,7 @@ fn serialize_deserialize() -> Result<(), ShellError> {
     let original_value = TestCustomValue(32);
     let span = Span::test_data();
     let serialized = PluginCustomValue::serialize_from_custom_value(&original_value, span)?;
-    assert_eq!(original_value.value_string(), serialized.name());
+    assert_eq!(original_value.type_name(), serialized.name());
     assert!(serialized.source.is_none());
     let deserialized = serialized.deserialize_to_custom_value(span)?;
     let downcasted = deserialized

crates/nu-plugin/src/protocol/test_util.rs

@@ -14,7 +14,7 @@ impl CustomValue for TestCustomValue {
         Value::custom_value(Box::new(self.clone()), span)
     }
 
-    fn value_string(&self) -> String {
+    fn type_name(&self) -> String {
         "TestCustomValue".into()
     }