[TextParagraph/Button] Add support for line spacing.

[bruvzg]

Fixes godotengine/godot-proposals#10476

Related proposal godotengine/godot-proposals#9778 (adds support to TextParagraph but not to all controls using it).

[timothyqiu]

What's the unit of line spacing? Also, I think we should make it clear in the documentation whether "line spacing" here means:

• The distance between adjacent baselines, or
• The distance between the bottom of one line and the top of the next.

[bruvzg]

What's the unit of line spacing?

Pixel.

The distance between adjacent baselines, or
The distance between the bottom of one line and the top of the next.

It's additional spacing, not a total spacing, so in does not matter from what point it's measured.

It's used in the same way existing line_spacing in Label/Label3D/TextMesh and line_separation in RichTextLabel work, spacing is added to the line descent:

    pos.y += line_ascent;
    draw_line(pos, ...); // draw use baseline
    pos.y += line_descent + line_spacing; 

Edit: Added some extra info to the documentation for all of these.

[Calinou]

Code looks good to me. It makes sense to expose this in Button since multiline buttons are fully supported in 4.x.

[Repiteo]

Thanks!