Used the yoast plugin icon as pinned plugin icon for gutenberg

[boblinthorst]

Summary

This PR can be summarized in the following changelog entry:

• Added the Yoast icon as pinned plugin icon in Gutenberg.

Relevant technical choices:

• Kept the styled component in edit.js because it's only a resize, which isn't used anywhere else.
• Set the size of the icon at 20x20px to match the other icons in the bar.

Test instructions

This PR can be tested by following these steps:

• Checkout this branch.
• Make sure the following flag is set : define( 'YOAST_FEATURE_GUTENBERG_SIDEBAR', true );
• Open a post-edit page.
• Make sure the yoast-sidebar is pinned ( open the yoast sidebar -> click on the star at the top ).
• Check if the icon added in the header toolbar by pinning yoast-seo is the yoast icon ( you can check the issue how it used to look for comparision ).

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended

Fixes #10295

[Dieterrr]

CR Done 👍
Acceptance done 👍