Add a browser check for AbortController

It's nice to have to aborting requests and adding timeouts to them.
This is in theory supported in all browsers covered by other checks,
but Safari 11.x had a broken implementation.

Fixes #38

packages/common/misc/browser-check.js

@@ -74,6 +74,13 @@ function supportsEval() {
     return true;
 }
 
+// https://caniuse.com/abortcontroller
+function supportsAbortController() {
+    // AbortController in older Safari is broken, so check for the signal property
+    // as well.
+    return (!!window.AbortController && Request.prototype.hasOwnProperty('signal'));
+}
+
 function isBrowserSupported() {
     if (!supportsEval()) {
         console.log("Eval support disabled, skipping browser feature detection.");
@@ -110,6 +117,11 @@ function isBrowserSupported() {
         return false;
     }
 
+    if (!supportsAbortController()) {
+        console.log("AbortController not supported");
+        return false;
+    }
+
     return true;
 }