fix: Typescript type of validator

[Laityned]

🐛 The bug
What isn't working? Describe what the bug is.

Currently I try to add a custom validation rule, in shorthand:
validation: (Rule) => Rule.custom((value, context) => true)

Custom Rules, should allow to obtain the context to reference other fields as described:
sanity docs
However, currently it gives an typescript error.

🛠️ To reproduce
Steps to reproduce the behavior:

🌈 Expected behaviour
What did you expect to happen? Is there a section in the docs about this?

Update the typescript definition to be compatible with sanity

ℹ️ Additional context

Thanks in advance