fix(require-hook): check variables are either const or declarations

[G-Rath]

While I documented it, I completely forgot to actually implement checking for this 🤦 (likewise for re-assignments, which I'll do in a follow-up PR).

Right now I'm not allowing initialization, but wouldn't be surprised if we supported that in the future since some people like doing that explicitly.

[G-Rath]

Ironically I just realised I'm one of those people that require initialization in the rare times when I'm writing vanilla Javascript, as a way of hinting what the expected type is 🤔

Unfortunately I don't think we can confidently confirm that a let variable is assigned to in a hook due to all the possible branching that could occur with nested describes (thought maybe it's acceptable if we say you should just be declaring the variable in each describe block? 🤔).

I still think it's a valuable check to have since the class of bugs it prevents tend to be very painful, so I think a good middle-ground that I'll do is to allow initialization to null or undefined only since those two values tend to be very static - you can't mutate them, and they are literally only two possible values (unlike other simple primitives e.g. numbers, which can be any number).

[SimenB]

works for me 👍

[github-actions]

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