ci: add renovate

Author: sapphi-red

.github/renovate.json

@@ -0,0 +1,5 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": ["github>Boshen/renovate", "helpers:pinGitHubActionDigestsToSemver"],
+  "ignorePaths": ["**/node_modules/**"]
+}