### 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)

tag_stack.push_front(tag);
		} else if (tag == "s") {
			
			push_strikethrough();
			pos = brk_end + 1;
			tag_stack.push_front(tag);
		} else if (tag == "lb") {
			add_text("[");
			pos = brk_end + 1;
		} else if (tag == "rb") {
			add_text("]");
			pos = brk_end + 1;
		} else if (tag == "lrm") {
			add_text(String::chr(0x200E));
			pos = brk_end + 1;
		} else if (tag == "rlm") {
			add_text(String::chr(0x200F));
			pos = brk_end + 1;
		} else if (tag == "lre") {
			add_text(String::chr(0x202A));
			pos = brk_end + 1;
		} else if (tag == "rle") {
			add_text(String::chr(0x202B));
			pos = brk_end + 1;
		} else if (tag == "lro") {
			add_text(String::chr(0x202D));
			pos = brk_end + 1;
		} else if (tag == "rlo") {
			add_text(String::chr(0x202E));
			pos = brk_end + 1;