Fixed #191

Used first code snippets from ClearURLs v2: https://gitlab.com/ClearURLs/core/-/blob/master/src/utils/URLHashParams.ts

core_js/utils/Multimap.d.ts

@@ -0,0 +1,21 @@
+/**
+ * Models a multimap backed by a {@link Set}.
+ */
+export default class Multimap<K, V> implements Iterable<[K, V]> {
+    private _map;
+    private _size;
+    constructor();
+    get size(): number;
+    get(key: K): Set<V>;
+    put(key: K, value: V): boolean;
+    has(key: K): boolean;
+    hasEntry(key: K, value: V): boolean;
+    delete(key: K): boolean;
+    deleteEntry(key: K, value: V): boolean;
+    clear(): void;
+    entries(): IterableIterator<[K, V]>;
+    values(): IterableIterator<V>;
+    keys(): IterableIterator<K>;
+    forEach<T>(callback: (this: T | this, key: K, value: V, map: this) => void, thisArg?: T): void;
+    [Symbol.iterator](): IterableIterator<[K, V]>;
+}