Add dependabot periodic dependency updates (#67)

wasm-tools and wit-bindgen are both quite active projects, and it's
not easy to update them after long periods. This commit adds a
dependabot configuration file to periodically update the dependencies
to align with the latest toolchain.

.github/dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: cargo
+    directory: /
+    schedule:
+      interval: monthly