### 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 UTF8_FLAG = 0x00000100;
	uint32_t string_block_len = decode_uint32(&r_manifest[16]);
	uint32_t string_count = decode_uint32(&r_manifest[20]);
	uint32_t string_flags = decode_uint32(&r_manifest[28]);
	const uint32_t string_table_begins = 40;
	Vector<String> string_table;
	String package_name = p_preset->get("package/name");
	Dictionary appnames = GLOBAL_GET("application/config/name_localized");
	for (uint32_t i = 0; i < string_count; i++) {
		uint32_t offset = decode_uint32(&r_manifest[string_table_begins + i * 4]);
		offset += string_table_begins + string_count * 4;
		String str = _parse_string(&r_manifest[offset], string_flags & UTF8_FLAG);
		if (str.begins_with("godot-project-name")) {