deps: change prop-types from peerDep to plain dep

[lbfrancisco]

prop-types aren't installed automatically when react-signature-canvas was installed.

[agilgur5]

prop-types has been a peerDep practically since it was spun out of React 6 years ago: #3.
So I'm not sure we should really change that without solid reason. Haven't heard a user complaint about it in those 6 years, even despite the ecosystem has changed how it handles peerDeps a few times now.

prop-types aren't installed automatically when react-signature-canvas was installed.

For instance, this behavior depends quite heavily on your package manager, and is not a bug of react-signature-canvas. If your package manager doesn't install peerDeps automatically, that is correct behavior.

It appears that the React team officially recommended it to be a dep shortly after its release though (facebook/prop-types#44), so it could be moved, but I'd still think it would be good to not have duplicate prop-types packages in a user's tree

[agilgur5]

This will be superseded by #125