### 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
uint32_t offset = 0;
	uint32_t len = 0;
	if (p_utf8) {
		uint8_t byte = p_bytes[offset];
		if (byte & 0x80) {
			offset += 2;
		} else {
			offset += 1;
		}
		byte = p_bytes[offset];
		offset++;
		if (byte & 0x80) {
			len = byte & 0x7F;
			len = (len << 8) + p_bytes[offset];
			offset++;
		} else {
			len = byte;
		}
	} else {
		len = decode_uint16(&p_bytes[offset]);