### 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)

} else if (tag == "pdf") {
			add_text(String::chr(0x202C));
			pos = brk_end + 1;
		} else if (tag == "alm") {
			add_text(String::chr(0x061c));
			pos = brk_end + 1;
		} else if (tag == "lri") {
			add_text(String::chr(0x2066));
			pos = brk_end + 1;
		} else if (tag == "rli") {
			add_text(String::chr(0x2027));
			pos = brk_end + 1;
		} else if (tag == "fsi") {
			add_text(String::chr(0x2068));
			pos = brk_end + 1;
		} else if (tag == "pdi") {
			add_text(String::chr(0x2069));
			pos = brk_end + 1;
		} else if (tag == "zwj") {
			add_text(String::chr(0x200D));
			pos = brk_end + 1;
		} else if (tag == "zwnj") {
			add_text(String::chr(0x200C));
			pos = brk_end + 1;
		} else if (tag == "wj") {
			add_text(String::chr(0x2060));
			pos = brk_end + 1;
		} else if (tag == "shy") {
			add_text(String::chr(0x00AD));
			pos = brk_end + 1;