utils/customError: narrow type of name property

The name property is inherited from Error and has a type of string.
But our CustomError ensures that the name is restricted to the type
of T. This causes compiler errors, so we had to enable a babel plugin
to support it.

src/utils/customError.ts

@@ -2,6 +2,8 @@
 // Copyright (c) 2022 The Pybricks Authors
 
 export class CustomError<T extends string> extends Error {
+    declare name: T;
+
     public constructor(name: T, message: string, cause?: Error) {
         super(message, { cause });
         this.name = name;