feat(bar): add update widget

This commit adds a new widget, "Update", which will check for komorebi
version updates using the cargo package version of the running binary
and the latest release returned from the GitHub API.

If the latest release is newer than the current cargo package version, a
widget will be shown, which can be clicked to open the changelog of the
latest release.

docs/komorebi.bar.example.json

@@ -33,6 +33,11 @@
     }
   ],
   "right_widgets": [
+    {
+      "Update": {
+        "enable": true
+      }
+    },
     {
       "Media": {
         "enable": true