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/Forward.h

@@ -208,6 +208,7 @@ class MessagePort;
 class PageTransitionEvent;
 class PromiseRejectionEvent;
 class SubmitEvent;
+class TextMetrics;
 class WebSocket;
 }
 
@@ -439,6 +440,7 @@ class SVGGraphicsElementWrapper;
 class SVGPathElementWrapper;
 class SVGSVGElementWrapper;
 class TextEncoderWrapper;
+class TextMetricsWrapper;
 class TextWrapper;
 class UIEventWrapper;
 class URLConstructor;