create temp config for custom testing

Author: TamaroWalter

.github/config.json

@@ -0,0 +1,15 @@
+{
+  "moodle-testmatrix": {
+    "MOODLE_405_STABLE": {
+      "php": ["8.1", "8.2", "8.3"]
+    },
+    "MOODLE_500_STABLE": {
+      "php": ["8.2", "8.3", "8.4"],
+      "db": ["pgsql", "mariadb", "mysqli"]
+    },
+    "MOODLE_501_STABLE": {
+      "php": ["8.2", "8.3", "8.4"],
+      "db": ["pgsql", "mariadb", "mysqli"]
+    }
+  }
+}