### Godot version

v4.2.dev4.official [549fcce5f]

### System information

Ubuntu 20.04 LTS

### Issue description

See title.

### Steps to reproduce

1. Create a new project
2. Create a csv file that includes the following
```
key,ar
key,placeholder
```
3. Add the generated `.translation` file to Project -> Project Settings... -> Localization
4. In Project -> Project Settings... -> Internationalization -> Locale, set `Test` to `ar`
5. Add a `LineEdit` node and type `-=[]{}`
6. Run the project, and notice that the text is on the right side, but the symbols are incorrectly in LTR (`-=[]{}`).  Correct output is `{}[]=-`
7. Back in the editor, add a single Arabic letter (any will do), and notice that `LineEdit` immediately (and correctly) flips the symbols, and the newly-added Arabic letter, to RTL.

**Notes**
- Identical behavior is seen with `TextEdit`.
- The purpose of steps 2-4 are to simulate an Arabic environment when the project is run, despite `OS.get_locale() == "en"` (or your system's current language).

### Minimal reproduction project

[rtlsymbols.zip](https://github.com/godotengine/godot/files/12525509/rtlsymbols.zip)
int64_t limit = p_pos;
	if (p_sd->text.length() != p_sd->utf16.length()) {
		const UChar *data = p_sd->utf16.get_data();
		for (int i = 0; i < p_pos; i++) {
			if (U16_IS_LEAD(data[i])) {
				limit--;
			}
		}
	}
	return limit;
}
int64_t TextServerAdvanced::_convert_pos_inv(const ShapedTextDataAdvanced *p_sd, int64_t p_pos) const {
	int64_t limit = p_pos;
	if (p_sd->text.length() != p_sd->utf16.length()) {
		for (int i = 0; i < p_pos; i++) {
			if (p_sd->text[i] > 0xffff) {
				limit++;
			}
		}
	}
	return limit;
}
void TextServerAdvanced::invalidate(TextServerAdvanced::ShapedTextDataAdvanced *p_shaped, bool p_text) {