### Godot version

3.5.1.stable, v4.0.beta.custom_build [0a87ead64]

### System information

Ubuntu 22.04.1

### Issue description

For example, if project has name "300spartans", we can't enter the "org.godotengine.300spartans" to "Unique Name" field. We get the error "Invalid package name: A digit cannot be the first character in a package segment". But , if we enter "org.godotengine.$genname", there will be no error message. When exporting, the name will be set as "org.godotengine.spartans" without "300" in beginning, but user will not be informed about it. I think, it is not very good. It may not be worth making changes to the source code, but mention it in the documentation.

### Steps to reproduce

See issue description.

### Minimal reproduction project

N/A
String pname = p_package;
	String basename = GLOBAL_GET("application/config/name");
	basename = basename.to_lower();
	String name;
	bool first = true;
	for (int i = 0; i < basename.length(); i++) {
		char32_t c = basename[i];
		if (is_digit(c) && first) {
			continue;
		}
		if (is_ascii_alphanumeric_char(c)) {
			name += String::chr(c);
			first = false;
		}
	}
	if (name.is_empty()) {
		name = "noname";
	}
	pname = pname.replace("$genname", name);
	return pname;
}
String EditorExportPlatformAndroid::get_assets_directory(const Ref<EditorExportPreset> &p_preset, int p_export_format) const {