AK: Add HashMap::ensure(key, callback)

This function ensures that a key is present in the HashMap.
If it's not present, it is inserted, and the corresponding value
is initialized with whatever the callback returns.

It allows us to express this:

    auto it = map.find(key);
    if (it == map.end()) {
        map.set(it, make_a_value());
        it = map.find(key);
    }
    auto& value = it->value;

Like this:

    auto& value = map.ensure(key, [] { return make_a_value(); });

Note that the callback is only invoked if we have to insert a missing
key into the HashMap. This is important in case constructing the default
value is expensive or otherwise undesirable.

AK/HashMap.h

@@ -139,6 +139,16 @@ public:
         return find(key)->value;
     }
 
+    template<typename Callback>
+    V& ensure(K const& key, Callback initialization_callback)
+    {
+        auto it = find(key);
+        if (it == end()) {
+            set(key, initialization_callback());
+        }
+        return find(key)->value;
+    }
+
     [[nodiscard]] Vector<K> keys() const
     {
         Vector<K> list;