Editor: add grid layout type

Add a new `grid` type to the layout block support.

Props andrewserong, costdev, ramonopoly.
Fixes #58554.


git-svn-id: https://develop.svn.wordpress.org/trunk@55975 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/theme.json

@@ -332,6 +332,28 @@
 							}
 						}
 					]
+				},
+				"grid": {
+					"name": "grid",
+					"slug": "grid",
+					"className": "is-layout-grid",
+					"displayMode": "grid",
+					"baseStyles": [
+						{
+							"selector": " > *",
+							"rules": {
+								"margin": "0"
+							}
+						}
+					],
+					"spacingStyles": [
+						{
+							"selector": "",
+							"rules": {
+								"gap": null
+							}
+						}
+					]
 				}
 			}
 		},