BREAKING CHANGE: Reactivity APIs adjustments:
- `readonly` is now non-tracking if called on plain objects.
`lock` and `unlock` have been removed. A `readonly` proxy can no
longer be directly mutated. However, it can still wrap an already
reactive object and track changes to the source reactive object.
- `isReactive` now only returns true for proxies created by `reactive`,
or a `readonly` proxy that wraps a `reactive` proxy.
- A new utility `isProxy` is introduced, which returns true for both
reactive or readonly proxies.
- `markNonReactive` has been renamed to `markRaw`.
