[BBT-94] Refactor UI Structure

Description

This PR updates the available tabs and refactors the style components so that they are reusable across site, blocks and elements.

Fixes BBT-94

Since the style components are all based on the schema reference for stylesProperties, they can be reused in multiple locations such as blocks, elements, elements within blocks and for the site styles themselves.

This PR is limited to site, blocks and elements. Elements within blocks should be possible but can be added after the UI has been improved to acommodate this better.

Change Log

Update main tab UI to Site, Blocks and Elements
Create Site component for site styles
Create Elements and ElementsItem component for element styles
Obtain list of elements from schema, same as we do for core blocks
Move individual Style... components from BlocksItem to a dedicated Style component
Use Style component inside BlocksItem, ElementsItem and Site, passing the path to where each style is located within the config

Steps to test

Load twentytwentythree theme.json - https://raw.githubusercontent.com/WordPress/twentytwentythree/trunk/theme.json

Paste this into the themer preview:

<!-- wp:pullquote -->
<figure class="wp-block-pullquote"><blockquote><p>"You miss 100% of the shots you don't take"<br>- <a href="https://www.bigbite.net">Wayne Gretzky</a></p><cite>Michael Scott</cite></blockquote></figure>
<!-- /wp:pullquote -->

Edit site styles
Edit core/pullquote block styles
Edit link element styles
Ensure styles update via the UI and are reflected in the preview and saved/exported theme.json

Screenshots/Videos

https://github.com/bigbite/themer/assets/41474928/e186f6c0-79c9-470a-8af3-e34998cd05b4

Checklist:

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

bigbite / themer