add box-shadow support for blocks via templates

[madhusudhand]

What?

This PR is an extension of #41972

It aims at two features

1. Define a set of shadows in theme.json similar to that of fontSizes and generate preset variables that can be used across the theme.

"settings": {
    "shadows": [{
            "slug": "material",
            "x": "5px",
            "y": "5px",
            "spread": "10px",
            "color": "green"
    },{
            "slug": "billboard",
            "x": "5px",
            "y": "5px",
            "spread": "10px",
            "color": "red"
    }],
}

2. Enable the use of slugs in templates

<!-- wp:post-title {"level":3,"isLink":true,"shadow":"material"} /-->

3. Enable the custom shadow (user selected values from the editor)

<!-- wp:post-title {"level":3,"isLink":true,"style":{"shadow": "10px 10px 1px 1px black"}} /-->

Why?

Changes #41972 helps in defining a shadow by targeting a block and it applies to all the instances.
While this is good, it doesn't allow to give a shadow only to a specific block such as apply shadow only to outer group block or special type of button etc..

This PR fills the gap by adopting the similar approach of fontSize

How?

Testing Instructions

Screenshots or screencast

[github-actions]

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[carolinan]

To eventually create an UI for this, does it not need both a name -the visual label, and the slug?