Add paragraph spacing for TextEdit

[ByteOfBrie]

Is your feature request related to a problem? Please describe.
I have a egui::TextEdit::multiline where like to configure paragraph spacing (how many pixels are after a \n in text). There doesn't seem to be a way to do this independently of line_height, which is entirely different.

Describe the solution you'd like
epaint::text::text_layout_types::TextFormat also getting a line_height option that specifically controls the height of newlines.

Describe alternatives you've considered
I tried to implement this manually in application code, so far unsuccessfully. I attempted to patch it in by setting line spacing at the start of every line (or to the newline character):
I've tried to add something like:

            job.append(
                "\n",
                0.0,
                TextFormat {
                    line_height: Some(40.0),
                    ..Default::default()
                },
            );

here

egui/crates/egui_demo_lib/src/easy_mark/easy_mark_highlighter.rs

Lines 117 to 121 in fdcaff8

	job.append(
	&text[..line_end],
	0.0,
	format_from_style(egui_style, &style),
	);

but a newline or empty string does nothing, and non-printable characters (i.e., \u200b) are printed as spaces anyway.

I think that I can make it work (mostly) by adjusting the line_height of the first actual character of every line. This will still be hacky with empty lines and the cursor UI, and will make application parsing code more complex to work around this, but it should be entirely possible.

Additional context
Originally opened #7357, before being increasingly confident that this doesn't exist, so this is being opened as a feature request.

I think the answer probably is somewhere in https://github.com/emilk/egui/blob/main/crates/epaint/src/text/text_layout.rs, but I'm not familiar enough with the code to find it.

I'm trying to work with code that's substantially similar to https://github.com/emilk/egui/blob/main/crates/egui_demo_lib/src/easy_mark/easy_mark_editor.rs, none of the differences are relevant here.

I've looked for similar issues, discussions, and even egui code that implements this, but I couldn't find anything. A pointer to somewhere else that implements this would be great too. I found a few commits referencing paragraph spacing such as 7336df5, but this was before the text layouting was rewritten in de1a1ba, so that was a dead end.

[ByteOfBrie]

I think that the code that would need to change to allow this is specifically galley_from_rows function:

egui/crates/epaint/src/text/text_layout.rs

Lines 643 to 726 in fdcaff8

	fn galley_from_rows(
	point_scale: PointScale,
	job: Arc<LayoutJob>,
	mut rows: Vec<PlacedRow>,
	elided: bool,
	intrinsic_size: Vec2,
	) -> Galley {
	let mut first_row_min_height = job.first_row_min_height;
	let mut cursor_y = 0.0;
	for placed_row in &mut rows {
	let mut max_row_height = first_row_min_height.at_least(placed_row.height());
	let row = Arc::make_mut(&mut placed_row.row);
	first_row_min_height = 0.0;
	for glyph in &row.glyphs {
	max_row_height = max_row_height.at_least(glyph.line_height);
	}
	max_row_height = point_scale.round_to_pixel(max_row_height);
	// Now position each glyph vertically:
	for glyph in &mut row.glyphs {
	let format = &job.sections[glyph.section_index as usize].format;
	glyph.pos.y = glyph.font_impl_ascent
	// Apply valign to the different in height of the entire row, and the height of this `Font`:
	+ format.valign.to_factor() * (max_row_height - glyph.line_height)
	// When mixing different `FontImpl` (e.g. latin and emojis),
	// we always center the difference:
	+ 0.5 * (glyph.font_height - glyph.font_impl_height);
	glyph.pos.y = point_scale.round_to_pixel(glyph.pos.y);
	}
	placed_row.pos.y = cursor_y;
	row.size.y = max_row_height;
	cursor_y += max_row_height;
	cursor_y = point_scale.round_to_pixel(cursor_y); // TODO(emilk): it would be better to do the calculations in pixels instead.
	}
	let format_summary = format_summary(&job);
	let mut rect = Rect::ZERO;
	let mut mesh_bounds = Rect::NOTHING;
	let mut num_vertices = 0;
	let mut num_indices = 0;
	for placed_row in &mut rows {
	rect |= placed_row.rect();
	let row = Arc::make_mut(&mut placed_row.row);
	row.visuals = tessellate_row(point_scale, &job, &format_summary, row);
	mesh_bounds |= row.visuals.mesh_bounds.translate(placed_row.pos.to_vec2());
	num_vertices += row.visuals.mesh.vertices.len();
	num_indices += row.visuals.mesh.indices.len();
	row.section_index_at_start = u32::MAX; // No longer in use.
	for glyph in &mut row.glyphs {
	glyph.section_index = u32::MAX; // No longer in use.
	}
	}
	let mut galley = Galley {
	job,
	rows,
	elided,
	rect,
	mesh_bounds,
	num_vertices,
	num_indices,
	pixels_per_point: point_scale.pixels_per_point,
	intrinsic_size,
	};
	if galley.job.round_output_to_gui {
	galley.round_output_to_gui();
	}
	galley
	}

I've also realized that #5784 removes that function and changes some of the internals of how text rendering works. I think it's probably best to try to figure out how to implement on top of that, rather than trying to add it in now and having to re-do it later, or creating more work for the PR itself. (At least, that's where any time/effort I spend on this will go)