Block Editor: Backport the Global Styles Variations endpoint.

This include the /global-styles/themes/{theme}/variations rest endpoint into core.
The endpoint will be used by the site editor to display alternative theme styles to the user.

Props gziolo, oandregal.
See #55505.


git-svn-id: https://develop.svn.wordpress.org/trunk@53072 602fd350-edb4-49c9-b593-d223f7449a82

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

@@ -0,0 +1,23 @@
+{
+    "version": 2,
+    "settings": {
+        "color": {
+            "palette": [
+                {
+                    "slug": "foreground",
+                    "color": "#3F67C6",
+                    "name": "Foreground"
+                }
+            ]
+        }
+    },
+    "styles": {
+        "blocks": {
+            "core/post-title": {
+                "typography": {
+                    "fontWeight": "700"
+                }
+            }
+        }
+    }
+}