Add renovate config

This config sets up renovate bot to:

* Run once per week
* Update Cargo.lock with the latest compatible versions. These PRs will be
  merged automatically if the tests pass.
* For dep updates that are already semver-compatible with the version in
  Cargo.toml, no changes will be made (except for the lockfile as described
  above). So, for example, we won't get PRs to update log-0.4.5 to 0.4.21 in
  Cargo.toml, because that's already semver-compat and covered by the lockfile
  change.
* For dep updates that are not semver-compat, a PR will be created.
* Updates of the `typos` action will be merged automatically if the tests pass,
  since that action gets updated a lot and is low-risk to merge without human
  oversight.

Author: renovate[bot]

.github/renovate.json

@@ -0,0 +1,18 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": [
+    "config:base",
+    "schedule:weekly"
+  ],
+  "lockFileMaintenance": {
+    "enabled": true,
+    "automerge": true
+  },
+  "rangeStrategy": "replace",
+  "packageRules": [
+    {
+      "matchPackageNames": ["crate-ci/typos"],
+      "automerge": true
+    }
+  ]
+}