dev: add warning when context is undefined

[Tommypop2]

Summary

Using useContext outside of a provider can cause unforeseen and cryptic errors. This PR adds a warning (in dev) when the result of useContext is falsy.

See https://discord.com/channels/722131463138705510/1358899310116213008

Typescript does protect against using an undefined context somewhat, but I thought a warning could be nice, especially for new users.

How did you test this change?

Created a project and consumed a context without being in its provider:

Warning is printed to the console.

After the project has been built, the warning is not logged.

[changeset-bot]

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Latest commit: bc6c19b

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