### Godot version

v4.0.2.stable.official [7a0977ce2]

### System information

Windows 10, not platform specific

### Issue description

Methods `get_character_line` and `get_character_paragraph` return wrong indices for characters that are first in the respective line/paragraph. See the attached screenshot.

![image](https://user-images.githubusercontent.com/892180/236546781-692603be-eb09-419e-822e-d4152a66d9b6.png)


### Steps to reproduce

Run attached project.

The script goes through all characters in `RichTextLabel` and prints the respective character, line index, and paragraph index. None of them should be -1 since we are iterating through valid indices.

### Minimal reproduction project

[RichTextLabel.zip](https://github.com/godotengine/godot/files/11409399/RichTextLabel.zip)

return Rect2(theme_cache.normal_style->get_offset(), get_size() - theme_cache.normal_style->get_minimum_size());
}
RichTextLabel::Item *RichTextLabel::_get_item_at_pos(RichTextLabel::Item *p_item_from, RichTextLabel::Item *p_item_to, int p_position) {
	int offset = 0;
	for (Item *it = p_item_from; it && it != p_item_to; it = _get_next_item(it)) {
		switch (it->type) {
			case ITEM_TEXT: {
				ItemText *t = static_cast<ItemText *>(it);
				offset += t->text.length();
				if (offset > p_position) {
					return it;
				}
			} break;
			case ITEM_NEWLINE: {
				offset += 1;
				if (offset == p_position) {
					return it;
				}
			} break;
			case ITEM_IMAGE: {
				offset += 1;
				if (offset > p_position) {
					return it;
				}
			} break;