Update theme.json classes for WordPress 5.9.

This commit ports to core the changes to the classes that deal with theme.json code.

See #54336.
Props oandregal, spacedmonkey, noisysocks, hellofromtonya, youknowriad.

git-svn-id: https://develop.svn.wordpress.org/trunk@52049 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/data/languages/themes/block-theme-pl_PL.po

@@ -22,6 +22,10 @@ msgctxt "Custom template name"
 msgid "Homepage template"
 msgstr "Szablon strony głównej"
 
+msgctxt "Template part name"
+msgid "Small Header"
+msgstr "Mały nagłówek"
+
 msgctxt "Color name"
 msgid "Light"
 msgstr "Jasny"

tests/phpunit/data/themedir1/block-theme-child/style.css

@@ -0,0 +1,8 @@
+/*
+Theme Name: Block Theme Child Theme
+Theme URI: https://wordpress.org/
+Description: For testing purposes only.
+Template: block-theme
+Version: 1.0.0
+Text Domain: block-theme-child
+*/

tests/phpunit/data/themedir1/block-theme-child/theme.json

@@ -0,0 +1,51 @@
+{
+	"version": 1,
+	"settings": {
+		"color": {
+			"palette": [
+				{
+					"slug": "light",
+					"name": "Light",
+					"color": "#f3f4f6"
+				},
+				{
+					"slug": "primary",
+					"name": "Primary",
+					"color": "#3858e9"
+				},
+				{
+					"slug": "dark",
+					"name": "Dark",
+					"color": "#111827"
+				}
+			],
+			"link": true
+		},
+		"blocks": {
+			"core/post-title": {
+				"color": {
+					"palette": [
+						{
+							"slug": "light",
+							"name": "Light",
+							"color": "#f3f4f6"
+						}
+					]
+				}
+			}
+		}
+	},
+	"customTemplates": [
+		{
+			"name": "page-home",
+			"title": "Homepage"
+		}
+	],
+	"templateParts": [
+		{
+			"name": "small-header",
+			"title": "Small Header",
+			"area": "header"
+		}
+	]
+}

tests/phpunit/data/themedir1/block-theme/theme.json

@@ -64,6 +64,7 @@
 	"templateParts": [
 		{
 			"name": "small-header",
+			"title": "Small Header",
 			"area": "header"
 		}
 	]