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.h

@@ -0,0 +1,66 @@
+/*
+ * Copyright (c) 2021, sin-ack <sin-ack@protonmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Heap/Handle.h>
+#include <LibWeb/Bindings/Wrappable.h>
+
+namespace Web::HTML {
+
+class TextMetrics
+    : public RefCounted<TextMetrics>
+    , public Bindings::Wrappable {
+public:
+    using WrapperType = Bindings::TextMetricsWrapper;
+
+    static RefPtr<TextMetrics> create();
+
+    double width() const { return m_width; }
+    double actual_bounding_box_left() const { return m_actual_bounding_box_left; }
+    double actual_bounding_box_right() const { return m_actual_bounding_box_right; }
+    double font_bounding_box_ascent() const { return m_font_bounding_box_ascent; }
+    double font_bounding_box_descent() const { return m_font_bounding_box_descent; }
+    double actual_bounding_box_ascent() const { return m_actual_bounding_box_ascent; }
+    double actual_bounding_box_descent() const { return m_actual_bounding_box_descent; }
+    double em_height_ascent() const { return m_em_height_ascent; }
+    double em_height_descent() const { return m_em_height_descent; }
+    double hanging_baseline() const { return m_hanging_baseline; }
+    double alphabetic_baseline() const { return m_alphabetic_baseline; }
+    double ideographic_baseline() const { return m_ideographic_baseline; }
+
+    void set_width(double width) { m_width = width; }
+    void set_actual_bounding_box_left(double left) { m_actual_bounding_box_left = left; }
+    void set_actual_bounding_box_right(double right) { m_actual_bounding_box_right = right; }
+    void set_font_bounding_box_ascent(double ascent) { m_font_bounding_box_ascent = ascent; }
+    void set_font_bounding_box_descent(double descent) { m_font_bounding_box_descent = descent; }
+    void set_actual_bounding_box_ascent(double ascent) { m_actual_bounding_box_ascent = ascent; }
+    void set_actual_bounding_box_descent(double descent) { m_actual_bounding_box_descent = descent; }
+    void set_em_height_ascent(double ascent) { m_em_height_ascent = ascent; }
+    void set_em_height_descent(double descent) { m_em_height_descent = descent; }
+    void set_hanging_baseline(double baseline) { m_hanging_baseline = baseline; }
+    void set_alphabetic_baseline(double baseline) { m_alphabetic_baseline = baseline; }
+    void set_ideographic_baseline(double baseline) { m_ideographic_baseline = baseline; }
+
+private:
+    explicit TextMetrics() { }
+
+    double m_width { 0 };
+    double m_actual_bounding_box_left { 0 };
+    double m_actual_bounding_box_right { 0 };
+
+    double m_font_bounding_box_ascent { 0 };
+    double m_font_bounding_box_descent { 0 };
+    double m_actual_bounding_box_ascent { 0 };
+    double m_actual_bounding_box_descent { 0 };
+    double m_em_height_ascent { 0 };
+    double m_em_height_descent { 0 };
+    double m_hanging_baseline { 0 };
+    double m_alphabetic_baseline { 0 };
+    double m_ideographic_baseline { 0 };
+};
+
+}