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/stor/create.rs

@@ -58,7 +58,7 @@ impl Command for StorCreate {
 
         process(table_name, span, &db, columns)?;
         // dbg!(db.clone());
-        Ok(Value::custom_value(db, span).into_pipeline_data())
+        Ok(Value::custom(db, span).into_pipeline_data())
     }
 }
 

crates/nu-command/src/stor/delete.rs

@@ -117,7 +117,7 @@ impl Command for StorDelete {
             }
         }
         // dbg!(db.clone());
-        Ok(Value::custom_value(db, span).into_pipeline_data())
+        Ok(Value::custom(db, span).into_pipeline_data())
     }
 }
 

crates/nu-command/src/stor/export.rs

@@ -73,7 +73,7 @@ impl Command for StorExport {
                 })?;
         }
         // dbg!(db.clone());
-        Ok(Value::custom_value(db, span).into_pipeline_data())
+        Ok(Value::custom(db, span).into_pipeline_data())
     }
 }
 

crates/nu-command/src/stor/import.rs

@@ -71,7 +71,7 @@ impl Command for StorImport {
                 })?;
         }
         // dbg!(db.clone());
-        Ok(Value::custom_value(db, span).into_pipeline_data())
+        Ok(Value::custom(db, span).into_pipeline_data())
     }
 }
 

crates/nu-command/src/stor/insert.rs

@@ -131,7 +131,7 @@ impl Command for StorInsert {
             };
         }
         // dbg!(db.clone());
-        Ok(Value::custom_value(db, span).into_pipeline_data())
+        Ok(Value::custom(db, span).into_pipeline_data())
     }
 }
 

crates/nu-command/src/stor/reset.rs

@@ -55,7 +55,7 @@ impl Command for StorReset {
                 })?;
         }
         // dbg!(db.clone());
-        Ok(Value::custom_value(db, span).into_pipeline_data())
+        Ok(Value::custom(db, span).into_pipeline_data())
     }
 }
 

crates/nu-command/src/stor/update.rs

@@ -143,7 +143,7 @@ impl Command for StorUpdate {
             };
         }
         // dbg!(db.clone());
-        Ok(Value::custom_value(db, span).into_pipeline_data())
+        Ok(Value::custom(db, span).into_pipeline_data())
     }
 }