Added header toolbar slotfill

[rinkalpagdar]

What?

Split from 16988

How?

This PR introduces a new SlotFill location at the end of the toolbar.

Testing Instructions

1. Checkout this branch and build
2. Add the sample code below in your theme or plugin
3. Confirm that a new icon appears and is clickable as seen in the screenshot

import { registerPlugin } from '@wordpress/plugins';
import { PluginHeaderToolbar } from '@wordpress/editor';

const MyHeaderToolbarPlugin = () => (
	<PluginHeaderToolbar
		className="plugin-header-toolbar-button"
		classContentName="plugin-header-toolbar-content"
		renderContent={ () => <div>Rendered content</div> }
	/>
);

registerPlugin( 'my-header-toolbar-plugin', {
	render: MyHeaderToolbarPlugin,
	icon: 'smiley',
} );

Screenshots or screencast

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: rinkalpagdar <[email protected]>
Co-authored-by: Mamaduka <[email protected]>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[github-actions]

👋 Thanks for your first Pull Request and for helping build the future of Gutenberg and WordPress, @rinkalpagdar! In case you missed it, we'd love to have you join us in our Slack community.

If you want to learn more about WordPress development in general, check out the Core Handbook full of helpful information.

[Mamaduka]

Thanks for contributing, @rinkalpagdar!

The linked issue already has a PR #52261. Please see the latest discussion there.

[rinkalpagdar]

Hello @Mamaduka thanks for the feedback. I have taken a look at PR #52261

There are suggesting this below change, that we have implemented. Please let me know if anything.

In case, we all agree we should proceed, the SlotFill will have to get moved to the @wordpress/editor package similar to what happened in the past with all other SlotFills so they can be used with all editors.

[Mamaduka]

@rinkalpagdar, currently, there's no clear path for allowing extensibility in the editor header. Please read the latest comment, which highlights that - #52261 (comment).

Until there's a better proposal, I would say the original issue and any associated PRs are on hold or blocked.