Add "disabled" property to ButtonStateful

[samwolfe-stripe]

Per the specification:https://developer.salesforce.com/docs/component-library/bundle/lightning-button-stateful/specification

ButtonStateful should have a disabled property

By not having this, we see this warning during tests:

[LWC warn]: Unknown public property "disabled" of element <lightning-button-stateful>. This is either a typo on the corresponding attribute "disabled", or the attribute does not exist in this browser or DOM implementation

[salesforce-cla[bot]]

Thanks for the contribution! Before we can merge this, we need @samwolfe-stripe to sign the Salesforce Inc. Contributor License Agreement.