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/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