AK+LibWeb: Use segmented vector to store commands in RecordingPainter

Using a vector to represent a list of painting commands results in many
reallocations, especially on pages with a lot of content.

This change addresses it by introducing a SegmentedVector, which allows
fast appending by representing a list as a sequence of fixed-size
vectors. Currently, this new data structure supports only the
operations used in RecordingPainter, which are appending and iterating.

Tests/AK/CMakeLists.txt

@@ -63,6 +63,7 @@ set(AK_TEST_SOURCES
     TestQuickSort.cpp
     TestRedBlackTree.cpp
     TestRefPtr.cpp
+    TestSegmentedVector.cpp
     TestSIMD.cpp
     TestSinglyLinkedList.cpp
     TestSlugify.cpp