Validate func on textinput doesn't restrict input

[daenmdir]

          Hi! So actually `textinput` contains a [`Validate`](https://pkg.go.dev/github.com/charmbracelet/bubbles@v0.18.0/textinput#Model) property which can be used to restrict input to numbers only. The [credit card example](https://github.com/charmbracelet/bubbletea/tree/master/examples/credit-card-form) illustrates how to do this.

Originally posted by @meowgorithm in #481 (comment)

This does not seem to be working as of v0.20.0. And I don't see a KeyMsg.Type for keyboard numbers to intercept the messages with non-numerical inputs.

Is there a way to easily circumvent this issue?