Set `ReactionContext`'s self to DeepReadonly to prevent misuse that will break the `deepEqual` comparison later

- In a reaction, old value is exposed as a non-readonly type
- Non-readonly type allows a reaction code to misuse the old value;
- One example of misuse is assigning the old value to a new value
- As a result, when doing a comparison between a stored old state and a new one, e.g. `newState.someprop == oldState.someprop`, it will return true `newState.someprop` has the same reference as `oldState`.

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Should this be solved with a programmatic approach: setting the type to Readonly?
Or, should this be solved with a "user manual" approach e.g. by providing FAQs, guide, etc?