### 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)

}
	
	Vector<int> subitem_indices_to_remove;
	for (int i = 0; i < current->subitems.size(); i++) {
		if (current->subitems[i]->line == p_paragraph) {
			subitem_indices_to_remove.push_back(i);
		}
	}
	bool had_newline = false;
	
	for (int i = subitem_indices_to_remove.size() - 1; i >= 0; i--) {
		int subitem_idx = subitem_indices_to_remove[i];
		had_newline = had_newline || current->subitems[subitem_idx]->type == ITEM_NEWLINE;
		_remove_item(current->subitems[subitem_idx], current->subitems[subitem_idx]->line, p_paragraph);
	}
	if (!had_newline) {
		current_frame->lines.remove_at(p_paragraph);
		if (current_frame->lines.size() == 0) {
			current_frame->lines.resize(1);
		}
	}
	if (p_paragraph == 0 && current->subitems.size() > 0) {
		main->lines[0].from = main;
	}
	int to_line = main->first_invalid_line.load();
	main->first_invalid_line.store(MIN(to_line, p_paragraph));
	queue_redraw();
	return true;
}
void RichTextLabel::push_dropcap(const String &p_string, const Ref<Font> &p_font, int p_size, const Rect2 &p_dropcap_margins, const Color &p_color, int p_ol_size, const Color &p_ol_color) {