Add box shadow support to blocks in templates

[madhudollu]

Bring the support for shadow with in templates.
Following are the proposed supports.

<!-- wp:post-title {"shadow":"natural"} /-->
<!-- wp:post-title {"style":{"shadow": "10px 10px 10px 10px #000"}} /-->

Note: The block must have the supports for "shadow" enabled for this to apply in the UI.

The solution has already been merged to Gutenberg here: ​https://github.com/WordPress/gutenberg/pull/46896

This ticket is to back-port these changes to the core.

[ramonopoly]

#58621 was marked as a duplicate.

This PR is a continuation of ​https://github.com/WordPress/wordpress-develop/pull/4664 but adds tests

props to @madhusudhand

It adds the PHP changes for the following Gutenberg PRs:

​https://github.com/WordPress/gutenberg/pull/46896

There are no front-end changes in this PR.

Testing instructions:

Enable supports for blocks such as post-title

src/wp-includes/blocks/post-title/block.json

{
...
"supports": {
  "shadow": true,
  ...
}

In the theme template add shadow attribute as follows.

Verify the post title UI to have the applied shadow.

Trac ticket: https://core.trac.wordpress.org/ticket/58590

[mukesh27]

@ramonopoly Left some feedback on PR. Set the milestone 6.3

Thank you for the review @mukeshpanchal27 ! 🙇

Thank you for adding the tests! @ramonjd

[isabel_brison]

In 56046:

Editor: add box shadow support to blocks.

Adds the ability for blocks to declare support for CSS box-shadow and processing of necessary styles.

Props madhudollu, sabernhardt, ramonopoly, spacedmonkey, mukesh27.
Fixes #58590.

Committed in r56046 / ​e55e9c2.