renovate: Enable automerge and schedule typos update monthly

Despite the name, automerge doesn't actually allow PRs to merge
automatically, because our repo settings require review for all
PRs. This just avoids needing to manually click merge after approving a
renovate PR.

Decrease `typos` update to once a month, since it's just a spellchecker.

Author: nicholasbishop

.github/renovate.json

@@ -6,5 +6,16 @@
   "lockFileMaintenance": {
     "enabled": true
   },
-  "rangeStrategy": "update-lockfile"
+  "rangeStrategy": "update-lockfile",
+  "automerge": true,
+  "packageRules": [
+    {
+      "matchPackageNames": [
+        "crate-ci/typos"
+      ],
+      "extends": [
+        "schedule:monthly"
+      ]
+    }
+  ]
 }