LibWeb: Display an IBeam cursor by default when mousing over a text node

[tcl3]

We now check the type of the layout node at the current mouse position when determining which cursor to display during a mouse move event.

Before:

https://github.com/SerenityOS/serenity/assets/2817754/3eaa886f-cb7b-409c-aa9f-52cb7b606ecf

After:

https://github.com/SerenityOS/serenity/assets/2817754/09a32b65-816f-48f5-bc29-755524be3ec0