Add experimental theme.json to seedlet

This change adds the experimental-theme.json to seedlet so that it can
properly style a link color in the Block Editor.

This JSON structure is correct for Gutenberg < 9.9 and will need to be
updated.

seedlet/experimental-theme.json

@@ -0,0 +1,9 @@
+{
+	"global": {
+		"styles": {
+			"color": {
+				"link": "var( --global--color-primary )"
+			}
+		}
+	}
+}