AK: Add StringBuilder::join() for joining collections with a separator

This patch adds a generic StringBuilder::join(separator, collection):

    Vector<String> strings = { "well", "hello", "friends" };
    StringBuilder builder;
    builder.join("+ ", strings);
    builder.to_string(); // "well + hello + friends"

AK/StringBuilder.h

@@ -56,6 +56,19 @@ public:
     bool is_empty() const { return m_length == 0; }
     void trim(size_t count) { m_length -= count; }
 
+    template<class SeparatorType, class CollectionType>
+    void join(const SeparatorType& separator, const CollectionType& collection)
+    {
+        bool first = true;
+        for (auto& item : collection) {
+            if (first)
+                first = false;
+            else
+                append(separator);
+            append(item);
+        }
+    }
+
 private:
     void will_append(size_t);