Fix: OCR block stops working on WP 5.7

[dinhtungdu]

Description of the Change

Due to changes of Gutenberg shipped in WP 5.7, the OCR block has been stopped working:

• The modal doesn't show up after inserting an image into the editor.
• The OCR text inserter inserts only group block, not the text inside it.

This PR fixes those issues by:

• Removing the usage of registerPlugin, moving the modal into the image control.
• Use await to ensure the text block is inserted only after the group block has been inserted into the editor.
• Simply the modal logic.

Verification Process

1. Setup and config the Image processing API, enable OCR feature.
2. Make sure the image is accessible from the Azure server.
3. Insert a new image that contains text, see:

• the modal shows up after the image has been inserted into the editor.
• Click on the Insert button, see a new group contains a paragraph with the text on the image as content.
• Click on the image, see a new disabled button (with clipboard icon).
• Delete the group block, click on the image again, see the clipboard button enabled.
• Click on that button, see the new description as using modal above.

Please check OCR functionality on both WP 5.6 and 5.7.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

Changelog Entry

[helen]

Minor typo, still need to test locally

[helen]

Works!