[suggestion] invalid discriminator value errors should include the given value, or signify if the value was omitted

[mitchell-merry]

Currently, a discriminated union schema, when given a value with a bad discriminator, does not spit out the given value in the ZodError:

const SomeSchema = z.discriminatedUnion('type', [
  z.object({ type: z.literal('foo'), foo: z.string() }),
  z.object({ type: z.literal('bar'), bar: z.string() }),
  z.object({ type: z.literal('baz'), baz: z.string() }),
]);

SomeSchema.parse({ type: 'qux' });

[
  {
    "code": "invalid_union_discriminator",
    "options": [
      "foo",
      "bar",
      "baz"
    ],
    "path": [
      "type"
    ],
    "message": "Invalid discriminator value. Expected 'foo' | 'bar' | 'baz'"
  }
]

It would be nice if this error included the invalid value.