node/basic: add a basic NodePrototypeChooser

[mvdan]

I keep seeing code like:

nodePrototypeChooser := dagpb.AddSupportToChooser(
	func(ipld.Link, ipld.LinkContext) (ipld.NodePrototype, error) {
		return basicnode.Prototype.Any, nil
	},
)

It's pretty common to stack chooser funcs on top of a fallback that just tries to use basicnode.Prototype.Any. If we defined a nice API for this, then the user could do:

nodePrototypeChooser := dagpb.AddSupportToChooser(basicnode.Chooser)

We could bikeshed on the name: Chooser, DefaultChooser, FallbackChooser. I think just "Chooser" is nice because it does just that - people could use it in a number of ways, it doesn't need to be a fallback or everyone's default.