Fix parse error for import() in older browsers (#571)

- add import() polyfill that evals in Function() and falls back to script tag injection
- add AbortController polyfill for MapLibre (lazy loaded so can be in bundle, not for Web Workers)
- support Firefox 56, last supporting old plugins before Web Extensions

.eslintrc.json

@@ -5,6 +5,6 @@
         "es2021": true
     },
     "settings": {
-        "polyfills": ["URL", "Promise", "navigator", "Uint8Array", "performance", "location.hostname"]
+        "polyfills": ["URL", "Promise", "navigator", "Uint8Array", "performance", "location.hostname", "document.body"]
     }
 }