Revert shadow.defaultPresets default to true

[ajlende]

Trac ticket: https://core.trac.wordpress.org/ticket/60815
WP Dev PR: WordPress/wordpress-develop#6295

What?

Reverts the core settings.shadow.defaultPresets back to true.

Why?

Fixes #59989

Themes should not change appearance with just a WP update. We have a migration mechanism for theme.json to allow for theme authors to change default values that should be used if we want to change a default.

All of the default* settings have been true by default as they've been added, so this brings consistency to that again as well.

How?

• Changes the settings.shadow.defaultPresets to true in lib/theme.json.
• Removes array( 'shadow', 'defaultPresets' ) from APPEARANCE_TOOLS_OPT_INS as it is no longer needed when we have true by default.
• Adds editor-shadow-presets and default-shadow-presets theme supports for classic themes to match how colors/gradients presets work.
• Updates related docs/tests.

Testing Instructions

Block themes

Here is an example theme.json for testing that would override the default natural shadow with a red shadow to make the difference obvious.

// theme.json
{
	"$schema": "https://schemas.wp.org/wp/6.4/theme.json",
	"version": 2,
	"settings": {
		"shadow": {
			"presets": [
				{
					"name": "Natural",
					"slug": "natural",
					"shadow": "6px 6px 9px #F00"
				}
			]
		},
		"layout": {
			"contentSize": "840px",
			"wideSize": "1100px"
		}
	},
	"styles": {
		"blocks": {
			"core/button": {
				"shadow": "var(--wp--preset--shadow--natural)"
			}
		}
	}
}

1. Use the above theme.json as an example theme.
2. Insert a button in a post.
3. Preview the post in wp/6.4
4. Preview the post in trunk
5. Preview the post in this PR
6. See that wp/6.4 and this PR match.

Classic themes

TODO: Finish writing testing scenarios.

• Test disabling shadow presets.

  add_theme_support( 'editor-shadow-presets', array() );

• Test custom shadow presets.

  add_theme_support( 'editor-shadow-presets', array(
    array(
      "name"   => "Custom Shadow",
      "slug"   => "custom-shadow",
      "shadow" => "10px 10px 0px 0px #F0F"
    )
  ) );

• Test disabling default shadow presets.

  add_theme_support( 'default-shadow-presets', false );

Testing Instructions for Keyboard

N/A

Screenshots or screencast

Block themes

WP 6.4

Current 6.5 ("defaultPresets": false)

This PR ("defaultPresets": true)

Classic themes

TODO: Screenshots of UI for classic themes in each of the testing scenarios.

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Co-authored-by: ajlende <ajlende@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: andrewserong <andrewserong@git.wordpress.org>
Co-authored-by: getdave <get_dave@git.wordpress.org>

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[ajlende]

Closing in favor of #60204