### 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)

_normalize_subtags(subtag);
				if (subtag.size() > 1) {
					if (subtag[0] == "top" || subtag[0] == "t") {
						alignment = INLINE_ALIGNMENT_TOP_TO;
					} else if (subtag[0] == "center" || subtag[0] == "c") {
						alignment = INLINE_ALIGNMENT_CENTER_TO;
					} else if (subtag[0] == "bottom" || subtag[0] == "b") {
						alignment = INLINE_ALIGNMENT_BOTTOM_TO;
					}
					if (subtag[1] == "top" || subtag[1] == "t") {
						alignment |= INLINE_ALIGNMENT_TO_TOP;
					} else if (subtag[1] == "center" || subtag[1] == "c") {
						alignment |= INLINE_ALIGNMENT_TO_CENTER;
					} else if (subtag[1] == "baseline" || subtag[1] == "l") {
						alignment |= INLINE_ALIGNMENT_TO_BASELINE;
					} else if (subtag[1] == "bottom" || subtag[1] == "b") {
						alignment |= INLINE_ALIGNMENT_TO_BOTTOM;
					}
				} else if (subtag.size() > 0) {
					if (subtag[0] == "top" || subtag[0] == "t") {
						alignment = INLINE_ALIGNMENT_TOP;
					} else if (subtag[0] == "center" || subtag[0] == "c") {
						alignment = INLINE_ALIGNMENT_CENTER;
					} else if (subtag[0] == "bottom" || subtag[0] == "b") {
						alignment = INLINE_ALIGNMENT_BOTTOM;
					}
				}
			}
			int end = p_bbcode.find("[", brk_end);
			if (end == -1) {