### 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
for (int j = 0; j < s.length(); j++) {
			encode_uint16(s[j], chars);
			chars += 2;
		}
		encode_uint16(0, chars);
		chars += 2;
	}
	for (int i = 0; i < stable_extra.size(); i++) {
		ret.push_back(stable_extra[i]);
	}
	
	while (ret.size() % 4) {
		ret.push_back(0);
	}
	uint32_t new_stable_end = ret.size();
	uint32_t extra = (p_manifest.size() - string_table_ends);
	ret.resize(new_stable_end + extra);
	for (uint32_t i = 0; i < extra; i++) {
		ret.write[new_stable_end + i] = p_manifest[string_table_ends + i];
	}
	while (ret.size() % 4) {
		ret.push_back(0);
	}
	encode_uint32(ret.size(), &ret.write[4]); 
	encode_uint32(new_stable_end - 8, &ret.write[12]); 
	encode_uint32(string_table.size(), &ret.write[16]); 
	encode_uint32(string_data_offset - 8, &ret.write[28]); 
	p_manifest = ret;
}
String EditorExportPlatformAndroid::_parse_string(const uint8_t *p_bytes, bool p_utf8) {