Fix Any unpacking to verify types

[timostamm]

The function anyUnpack is documented as:

Unpacks the message the Any represents.
Returns undefined if the Any is empty, or if it does not contain the type given by schema.

But instead of returning undefined, the function attempts to unpack an Any that contains a different type:

import {anyPack, anyUnpack} from "@bufbuild/protobuf/wkt";
import {create} from "@bufbuild/protobuf";
import {FooSchema, UserSchema} from "./gen/example_pb";

const user = create(UserSchema, {
  firstName: "John",
});
const any = anyPack(UserSchema, user);
const msg1 = anyUnpack(any, UserSchema); // ok
const msg2 = anyUnpack(any, FooSchema); // runtime error

anyUnpackTo has the same issue. This PR fixes the bug, and adds test coverage.