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

[Kelerchian]

• 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?