decode function for keyed varients

I think with the current API the expected way to solve varients is using `one_of` this doesn't give very good errors.
I have been using the following function in several projects. It decodes first a discriminator then picks a subsequent decoder based on it.
This allows it to show a specific error for the discriminator, and then good errors if the discriminator is known but it still fails to decode.

Here is the function https://github.com/CrowdHailer/oas/blob/main/src/oas/decodex.gleam#L3

Check the tests for using it and the errors it gives https://github.com/CrowdHailer/oas/blob/main/test/oas/decodex_test.gleam

I'm sure there's an argument for not extending the standard library, in which case I'd be interested in the best way to define a decoder (just a decoder without an essentially similar discriminate function extracted) that gives useful errors.

The proposal in this issue is to add a discriminate function the same or similar to the one I have linked here