Minimal editable text scrolling implementation

[ickshonpe]

Objective

Add basic support for scrolling text horizontally and vertically to EditableText.

Solution

• New component TextScroll. Wraps a Vec2 offset that can be used to set the scroll position.
• Clip overflowing editable text content automatically.
• New system scroll_editable_text sets TextScroll to keep the cursor within view inside the EditableText's content box.
• New allow_newline flag on EditableText. Allows insertion of newlines with the enter key.
• editable_text_system previously ignored TextLayout::linebreak. Now it's possible to change the line break settings.
• Scrolling only happens after a TextEdit is applied.

The implementation here is extremely simple but it has some flaws:

• Depending on the width of the node sometimes the cursor can get clipped.
• allow_newlines: false doesn't block pasting in a section of text with a newline
• No support for scroll margins yet.
• Drag-to-scroll seems to work, but I'm not super confident in it and it may need some changes.

Went back and forth over whether ScrollPosition should be used instead of a dedicated component, I think it's probably simpler with TextScroll but not certain. Clipping doesn't use Node's overflow API and is handled automatically during extraction, always targeting the content box.

Testing

cargo run --example multiline_text_input

[Zeophlite]

So I've noticed if you add enough spaces, it will scroll a have character width to the right. Either dragging with the pointer, or just using the left or right cursors can reset it. Seems a little annoying if not intentional, but also I can see it being useful for long text.

[Zeophlite]

LGTM