Add type="button" by default to Button component

[kevin940726]

<Button> component should have type="button" by default, or else it will default to type="submit" when placing inside a <form> element. It's also the default value in gutenberg's <Button> component: https://github.com/WordPress/gutenberg/blob/trunk/packages/components/src/button/index.js#L77

Alternatively, we can enforce explicit type attribute with tools like eslint.

I ran into this problem when trying to place some components here inside a <form> element: WordPress/gutenberg#29755 (comment). IMHO, this should be handled in the design system rather in the app level.

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