Rename Value::CustomValue to Value::Custom (#12309)

# Description
The second `Value` is redundant and will consume five extra bytes on
each transmission of a custom value to/from a plugin.

# User-Facing Changes
This is a breaking change to the plugin protocol.

The [example in the protocol
reference](https://www.nushell.sh/contributor-book/plugin_protocol_reference.html#value)
becomes

```json
{
  "Custom": {
    "val": {
      "type": "PluginCustomValue",
      "name": "database",
      "data": [36, 190, 127, 40, 12, 3, 46, 83],
      "notify_on_drop": true
    },
    "span": {
      "start": 320,
      "end": 340
    }
  }
}
```

instead of 

```json
{
  "CustomValue": {
    ...
  }
}
```


# After Submitting
Update plugin protocol reference

crates/nu-command/src/conversions/into/string.rs

@@ -226,7 +226,7 @@ fn action(input: &Value, args: &Arguments, span: Span) -> Value {
             },
             span,
         ),
-        Value::CustomValue { val, .. } => {
+        Value::Custom { val, .. } => {
             // Only custom values that have a base value that can be converted to string are
             // accepted.
             val.to_base_value(input.span())