feat: add loading state to button

[joshuagraber]

Add loading UI to button component

Background

• Rather than repeat loading state with every instance, we add it here.

Description

• Adds loading UI to button when isLoading prop is truthy
• Spinner is default, but it will render slot content instead if passed.

Acceptance Criteria

1. Run demo app, hard code isLoading prop of one of the buttons to be true, observe.

[github-actions[bot]]

:tada: This PR is included in version 3.1.0-beta.14 :tada:

The release is available on:

• npm package (@beta dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: