Transform mixed to unknown in xplat/js (#54955)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/54955

We are transforming the following utility types to be more consistent with typescript and better AI integration:

* `$NonMaybeType` -> `NonNullable`
* `$ReadOnly` -> `Readonly`
* `$ReadOnlyArray` -> `ReadonlyArray`
* `$ReadOnlyMap` -> `ReadonlyMap`
* `$ReadOnlySet` -> `ReadonlySet`
* `$Keys` -> `keyof`
* `$Values` -> `Values`
* `mixed` -> `unknown`

See details in https://fb.workplace.com/groups/flowlang/permalink/1837907750148213/.
drop-conflicts

Reviewed By: SamChou19815

Differential Revision: D89581584

fbshipit-source-id: cc5953ffea6cf1060f69efc714508e8ba93a040c