### 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
encode_uint32(-1, &p_manifest.write[ofs + 12]); 
						encode_uint32(-1, &p_manifest.write[ofs + 16]); 
						encode_uint32(attr_uses_permission_string, &p_manifest.write[ofs + 20]); 
						ofs += 24;
					}
					
					memcpy(&p_manifest.write[ofs], manifest_end.ptr(), manifest_end.size());
				}
			} break;
		}
		ofs += size;
	}
	
	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] = p_manifest[i];
	}
	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);
	string_data_offset = ret.size() - ofs;
	uint8_t *chars = &ret.write[string_data_offset];
	for (int i = 0; i < string_table.size(); i++) {
		String s = string_table[i];
		encode_uint16(s.length(), chars);
		chars += 2;