[new] add `PropTypes.literal`

[wojtekmaj]

Currently, to check if value is exactly something, we need to do it in a way that feels a bit hacky:

PropTypes.oneOf([value])

With this proposal, one could use:

PropTypes.literal(value)

to achieve the same thing.

[ljharb]

I'm not sure the benefit here is worth the complexity of adding an entirely new propType.