ci: add renovate

Author: sapphi-red

.github/renovate.json

@@ -0,0 +1,18 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": ["config:recommended", "schedule:weekly", "group:allNonMajor"],
+  "labels": ["dependencies"],
+  "ignorePaths": ["**/__tests__/**"],
+  "rangeStrategy": "bump",
+  "packageRules": [
+    {
+      "matchDepTypes": ["action"],
+      "pinDigests": true,
+      "matchPackageNames": ["!actions/{/,}**", "!github/{/,}**"],
+    },
+  ],
+  "ignoreDeps": [
+    // manually bumping
+    "node",
+  ],
+}