Adds `Dependabot` configuration file (#676)

* Adds `cargo` dependabot checks for dependencies

- Allows dependabot to automatically check for dependency version
updates daily using cargo.
- `versioning-strategy` supports default strategy and lockfile-only for
cargo, will need to see which is more suitable.
- Perhaps add `rebase-strategy` and `reviewers` if necessary, although
with proper unit tests, shouldn't be too necessary.

* Adds github-actions updates to dependabot

.github/dependabot.yml

@@ -0,0 +1,27 @@
+---
+version: 2
+updates:
+        - package-ecosystem: "npm"
+          directory: "/"
+          schedule:
+                  interval: "daily"
+        - package-ecosystem: "github-action"
+          directory: "/"
+          schedule:
+                  interval: "daily"
+        - package-ecosystem: "cargo"
+          directory: "/"
+          schedule:
+                  interval: "daily"
+        - package-ecosystem: "cargo"
+          directory: "/boa/"
+          schedule:
+                  interval: "daily"
+        - package-ecosystem: "cargo"
+          directory: "/boa_cli/"
+          schedule:
+                  interval: "daily"
+        - package-ecosystem: "cargo"
+          directory: "/boa_wasm/"
+          schedule:
+                  interval: "daily"