[BBT-53] Add a new ButtonExport component and corresponding rest endpoint

[chrishbite]

Description

Fixes BBT-53

Allows a user to export an updated theme.json file with the layers merged and flattened.

The ButtonExport component is reusable and can be placed anywhere, for this PR it has been added next to the other buttons.

<ButtonExport />

Change Log

• A new reusable ButtonExport component
• New **/themer/v1/export rest route

Steps to test

Notes: As the main UI for themer isn't implemented yet, suggest to use the Site Editor for the below testing steps to check functionality of the export on the TwentyTwenty-Three theme.

1. Use the Reset to defaults button to reset all user styles/settings and save
2. Add multiple custom colours via Styles > Colours > Palette
3. Set Styles > Colours > Colours > * options to random colours from the palette
4. Set a couple of Styles > Typography > Elements to random settings
5. Change Styles > Blocks > Paragraph options
6. Save
7. Click the Export theme.json button on the Themer admin page
8. Save the file to your machine
9. Use the Reset to defaults button to reset all user styles/settings again and save
10. Refresh both Themer and Site Editor pages (they should now be back to default styles from the themes theme.json)
11. Open up the twentytwentythree/theme.json file and replace with exported files content
12. Refresh both Themer and Site Editor pages
13. Confirm that the Settings now reflect the changes made previously and were exported

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

[g-elwell]

This works great, my only request is that we update all the class usage in class-rest-api.php to use the same format. Happy to go with the declarative approach and avoid \ if you don't mind updating the other class references in this file?

[chrishbite]

This works great, my only request is that we update all the class usage in class-rest-api.php to use the same format. Happy to go with the declarative approach and avoid \ if you don't mind updating the other class references in this file?

Updated those usages and references @g-elwell 👍

[g-elwell]

@chrishbite cheers! Just noticed this one is targeting main, can you update to target release/1.0.0 ?

[g-elwell]

LGTM