LibWeb: Implement CanvasRenderingContext2D.measureText

This requires an implementation of the "text preparation algorithm" as
specified here:

html.spec.whatwg.org/multipage/canvas.html#text-preparation-algorithm

However, we're missing a lot of things such as the
CanvasTextDrawingStyles interface, so most of the algorithm was not
implemented. Additionally, we also are not able to use a LineBox like
the algorithm suggests, because our layouting infra is not up to the
task yet. The prepare_text function does nothing other than figuring out
the width of the given text and return glyphs with offsets at the
moment.

Userland/Libraries/LibWeb/HTML/TextMetrics.cpp

@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) 2021, sin-ack <sin-ack@protonmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "TextMetrics.h"
+
+namespace Web::HTML {
+
+RefPtr<TextMetrics> TextMetrics::create()
+{
+    return adopt_ref(*new TextMetrics());
+}
+
+}