### 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 scroll_active;
}
void RichTextLabel::set_scroll_follow(bool p_follow) {
	scroll_follow = p_follow;
	if (!vscroll->is_visible_in_tree() || vscroll->get_value() >= (vscroll->get_max() - vscroll->get_page())) {
		scroll_following = true;
	}
}
bool RichTextLabel::is_scroll_following() const {
	return scroll_follow;
}
void RichTextLabel::parse_bbcode(const String &p_bbcode) {
	clear();
	append_text(p_bbcode);
}
void RichTextLabel::append_text(const String &p_bbcode) {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	int pos = 0;
	List<String> tag_stack;
	int indent_level = 0;
	bool in_bold = false;
	bool in_italics = false;
	bool after_list_open_tag = false;
	bool after_list_close_tag = false;
	set_process_internal(false);
	while (pos <= p_bbcode.length()) {
		int brk_pos = p_bbcode.find("[", pos);
		if (brk_pos < 0) {
			brk_pos = p_bbcode.length();