### 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
if (str == "godot-project-name") {
				
				str = get_project_name(package_name);
			} else {
				String lang = str.substr(str.rfind("-") + 1, str.length()).replace("-", "_");
				if (appnames.has(lang)) {
					str = appnames[lang];
				} else {
					str = get_project_name(package_name);
				}
			}
		}
		string_table.push_back(str);
	}
	
	Vector<uint8_t> ret;
	ret.resize(string_table_begins + string_table.size() * 4);
	for (uint32_t i = 0; i < string_table_begins; i++) {
		ret.write[i] = r_manifest[i];
	}
	int ofs = 0;
	for (int i = 0; i < string_table.size(); i++) {
		encode_uint32(ofs, &ret.write[string_table_begins + i * 4]);
		ofs += string_table[i].length() * 2 + 2 + 2;
	}
	ret.resize(ret.size() + ofs);
	uint8_t *chars = &ret.write[ret.size() - ofs];
	for (int i = 0; i < string_table.size(); i++) {
		String s = string_table[i];
		encode_uint16(s.length(), chars);