Adds support for custom variations

[orballo]

Summary

This PR adds support for custom variations in FSE. If there is no option extendable_custom_variation in the database, it won't add a custom variation to the menus, however, if the extendable_custom_variation is populated, it will render the custom variation, which behaves like any other variation.

For testing you can crate the option extendable_custom_variation and set this JSON as the value:

{
	"$schema": "https://schemas.wp.org/trunk/theme.json",
	"title": "Custom Variation (DB)",
	"version": 2,
	"settings": {
		"color": {
			"duotone": [
				{
					"colors": ["#FF0000", "#B22222"],
					"slug": "primary-and-secondary",
					"name": "Primary and secondary"
				},
				{
					"colors": ["#FF0000", "#FFD700"],
					"slug": "primary-and-tertiary",
					"name": "Primary and tertiary"
				},
				{
					"colors": ["#B22222", "#FF0000"],
					"slug": "secondary-and-primary",
					"name": "Secondary and primary"
				},
				{
					"colors": ["#000000", "#FFFFFF"],
					"slug": "foreground-and-background",
					"name": "Foreground and background"
				},
				{
					"colors": ["#FF0000", "#000000"],
					"slug": "primary-foreground",
					"name": "Primary and Foreground"
				}
			],
			"palette": [
				{
					"slug": "foreground",
					"color": "#000000",
					"name": "Foreground"
				},
				{
					"slug": "background",
					"color": "orange",
					"name": "Background"
				},
				{
					"slug": "primary",
					"color": "#FF0000",
					"name": "Primary"
				},
				{
					"slug": "secondary",
					"color": "#B22222",
					"name": "Secondary"
				},
				{
					"slug": "tertiary",
					"color": "#FFD700",
					"name": "Tertiary"
				},
				{
					"slug": "foreground-alt",
					"color": "#800000",
					"name": "Foreground Alt"
				}
			]
		},
		"custom": {
			"elements": {
				"button": {
					"border": {
						"radius": "0.4rem"
					}
				}
			}
		}
	},
	"styles": {
		"blocks": {
			"core/button": {
				"variations": {
					"outline": {
						"spacing": {
							"padding": {
								"top": "calc(0.838rem - 1px)",
								"right": "calc(2.5rem - 1px)",
								"bottom": "calc(0.838rem - 1px)",
								"left": "calc(2.5rem - 1px)"
							}
						}
					}
				}
			}
		},
		"color": {
			"text": "var(--wp--preset--color--foreground-alt)"
		},
		"elements": {
			"button": {
				"border": {
					"radius": "var(--wp--custom--elements--button--border--radius)"
				},
				"spacing": {
					"padding": {
						"top": "0.838rem",
						"right": "2.5rem",
						"bottom": "0.838rem",
						"left": "2.5rem"
					}
				},
				"typography": {
					"fontSize": "1.0625rem",
					"fontWeight": "var(--wp--custom--typography--font-weight--medium)",
					"lineHeight": "inherit"
				}
			},
			"h1": {
				"typography": {
					"fontWeight": "400"
				}
			},
			"h2": {
				"typography": {
					"fontWeight": "400"
				}
			},
			"h3": {
				"typography": {
					"fontWeight": "400"
				}
			},
			"h4": {
				"typography": {
					"fontWeight": "400"
				}
			},
			"h5": {
				"typography": {
					"fontWeight": "400"
				}
			},
			"h6": {
				"typography": {
					"fontWeight": "400"
				}
			},
			"heading": {
				"color": {
					"text": "var(--wp--preset--color--foreground)"
				},
				"typography": {
					"fontFamily": "var(--wp--preset--font-family--marcellus)"
				}
			}
		},
		"typography": {
			"fontFamily": "var(--wp--preset--font-family--ibm-plex-sans)"
		}
	}
}

This JSON is a clone of the Bloom variation but with the colors changed.