[types/EuiButtonEmpty] add missing isLoading and href props

[spalger]

Tried to use <EuiButtonEmpty /> in typescript but the href prop wasn't typed, pulled over the isLoading prop too because it seemed to be missing but is defined in the propTypes

[cchaos]

Looks right to me. Would you mind taking it a bit further and adding those props to the other 2 button types as well?

@chandlerprall Any idea why onClick would not have been added to EuiButtonEmptyProps originally and if we should go ahead and add it now?

@spalger Also, be sure to add a changelog entry in CHANGELOG.md (you should just be able to follow the pattern).

[chandlerprall]

@cchaos onClick is covered by combining with ButtonHTMLAttributes further down in the file

[chandlerprall]

Needs bugfix entry to CHANGELOG.md

[spalger]

@chandlerprall thanks, updated the changelog, removed the requirement for onClick prop since an href without an onClick is totally valid, and added isLoading prop to get them all in sync.

[spalger]

So, the latest types I supplied are somewhat undesirable because it changes the type of the onClick() prop to require a handler for React.MouseEvent<HTMLButtonElement | HTMLAnchorElement>, which really isn't necessary if people are relying solely on onClick() and using this as a standard button. I've tried a couple different ways of having TypeScript detect if you're using the href attribute and base onClick() on that, but I haven't figured out a way to do that which doesn't have other negative side effects... Going to try a couple other things but this will probably impact the default <EuiButton /> use case more than it's worth...

[spalger]

Okay, these types work for the following situations:

<EuiButton
  href="string"
  onClick={(e: React.MouseEvent<HTMLAnchorElement>) => undefined}
/>

<EuiButton
  onClick={(e: React.MouseEvent<HTMLButtonElement>) => undefined}
/>

<EuiButton
  href={optionalHref}
  onClick={(e: React.MouseEvent<HTMLButtonElement | HTMLAnchorElement>) => undefined}
/>

[chandlerprall]

Changes LGTM. Nice solution @spalger !