### 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
const int CHUNK_STRINGS = 0x001C0001;
	
	const int CHUNK_XML_END_TAG = 0x00100103;
	
	const int CHUNK_XML_START_TAG = 0x00100102;
	
	const int UTF8_FLAG = 0x00000100;
	Vector<String> string_table;
	uint32_t ofs = 8;
	uint32_t string_count = 0;
	uint32_t string_flags = 0;
	uint32_t string_data_offset = 0;
	uint32_t string_table_begins = 0;
	uint32_t string_table_ends = 0;
	Vector<uint8_t> stable_extra;
	String version_name = p_preset->get("version/name");
	int version_code = p_preset->get("version/code");
	String package_name = p_preset->get("package/unique_name");
	const int screen_orientation =
			_get_android_orientation_value(DisplayServer::ScreenOrientation(int(GLOBAL_GET("display/window/handheld/orientation"))));
	bool screen_support_small = p_preset->get("screen/support_small");
	bool screen_support_normal = p_preset->get("screen/support_normal");
	bool screen_support_large = p_preset->get("screen/support_large");