feat: Add sessions namespace to metadata

https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/sessions#Browser_compatibility

Only `sessions.getRecentlyClosed` and `sessions.restore` are currently
supported by Firefox.

`sessions.getDevices` is included anyway since this method may be added
to Firefox in the future: https://bugzil.la/1308059

api-metadata.json

@@ -422,6 +422,20 @@
       "maxArgs": 1
     }
   },
+  "sessions": {
+    "getDevices": {
+      "minArgs": 0,
+      "maxArgs": 1
+    },
+    "getRecentlyClosed": {
+      "minArgs": 0,
+      "maxArgs": 1
+    },
+    "restore": {
+      "minArgs": 0,
+      "maxArgs": 1
+    }
+  },
   "storage": {
     "local": {
       "clear": {