Greasemonkey: mock the new GM4 promises based API.

Based on the gm4-polyfill.js script from the greasemonkey devs. But not
the same because that script doesn't work for us for a couple of
reasons:

* It assumes all GM_* functions are attributes of `this` which in
this case is the global window object. Which breaks it out of our iife.
It is possible to change what `this` is within the iife but then we
would have to do something weird to ensure the functions were available
with the leading `this.`. And I don't think user javascripts tend to
call GM functions like that anyway, that polyfill script is just making
weird assumptions and then claiming it'll work for "any user script
engine".

* It tries to provide implementations of GM_registerMenuCommand and
GM_getResource text which do unexpected thins or implement a circular
dependency on the new version, respectively.

qutebrowser/javascript/greasemonkey_wrapper.js

@@ -110,6 +110,42 @@
         }
     }
 
+    // Stub these two so that the gm4 polyfill script doesn't try to
+    // create broken versions as attributes of window.
+    function GM_getResourceText(caption, commandFunc, accessKey) {
+        console.error(`${GM_info.script.name} called unimplemented GM_getResourceText`);
+    }
+
+    function GM_registerMenuCommand(caption, commandFunc, accessKey) {
+        console.error(`${GM_info.script.name} called unimplemented GM_registerMenuCommand`);
+    }
+
+    // Mock the greasemonkey 4.0 async API.
+    const GM = {};
+    GM.info = GM_info;
+    Object.entries({
+        'log': GM_log,
+        'addStyle': GM_addStyle,
+        'deleteValue': GM_deleteValue,
+        'getValue': GM_getValue,
+        'listValues': GM_listValues,
+        'openInTab': GM_openInTab,
+        'setValue': GM_setValue,
+        'xmlHttpRequest': GM_xmlhttpRequest,
+    }).forEach(([newKey, old]) => {
+        if (old && (typeof GM[newKey] == 'undefined')) {
+            GM[newKey] = function(...args) {
+                return new Promise((resolve, reject) => {
+                    try {
+                        resolve(old(...args));
+                    } catch (e) {
+                        reject(e);
+                    }
+                });
+            };
+        }
+    });
+
     const unsafeWindow = window;
 
     // ====== The actual user script source ====== //