fix: Fix downloads schemas

- Alphabetically sort keys
- Add `"fallbackToNoCallback": true` to `open` and `show`
  because these methods do currently not take any callbacks in Chrome:
  https://developer.chrome.com/extensions/downloads#method-open
  https://developer.chrome.com/extensions/downloads#method-show
- Chrome has a `downloads.acceptDanger` method, but Firefox does not,
  so this is not included in the schema.

api-metadata.json

@@ -219,11 +219,11 @@
     }
   },
   "downloads": {
-    "download": {
+    "cancel": {
       "minArgs": 1,
       "maxArgs": 1
     },
-    "cancel": {
+    "download": {
       "minArgs": 1,
       "maxArgs": 1
     },
@@ -237,7 +237,8 @@
     },
     "open": {
       "minArgs": 1,
-      "maxArgs": 1
+      "maxArgs": 1,
+      "fallbackToNoCallback": true
     },
     "pause": {
       "minArgs": 1,
@@ -257,7 +258,8 @@
     },
     "show": {
       "minArgs": 1,
-      "maxArgs": 1
+      "maxArgs": 1,
+      "fallbackToNoCallback": true
     }
   },
   "extension": {