### 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 aname;
	if (!p_name.is_empty()) {
		aname = p_name;
	} else {
		aname = GLOBAL_GET("application/config/name");
	}
	if (aname.is_empty()) {
		aname = VERSION_NAME;
	}
	return aname;
}
String EditorExportPlatformAndroid::get_package_name(const String &p_package) const {