LibWeb: Use GlyphRun fonts, not the first available font, for more things

[AtkinsSJ]

Partly addresses #2787.

I found a few places where we're calling first_available_font() for things where we actually want to know the exact font used by these particular code-points. And, I've fixed some. Specifically, selection rectangles and text-editing cursor positions.

Before

Screencast_20241205_200256.webm

After

Screencast_20241205_200448.webm

Source

<style>
html, textarea { font-family: lololol; }
</style>
<textarea>Well hello friends! 😅😅😅😅😅😅😅😅😅😅</textarea>

It is absolutely not perfect still. A couple of issues:

1. Somehow, Noto Emoji has a space character, and since that's the first font, that's the space character we use - hence the comically large spaces between words.
2. Selecting multiple emojis near the end of the textarea still jumps the selection around. But it's better than it was.

The remaining spots we don't handle, which I've FIXME'd:

• SVG text paths just use the first available font, instead of identifying which font per code-point like regular text painting.
• The text in list-item markers use the first available font too.

(There are also several places where using the first available font is appropriate as far as I can tell, so I've left those.)

We might have other places we do the wrong thing like this, but I'm sure we'll notice those eventually.