### 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
string_table.push_back(itos(feature_version));
									version_value = string_table.size() - 1;
								}
								tag_size = 96; 
								attr_count = 3;
							} else {
								tag_size = 76; 
								attr_count = 2;
							}
							manifest_cur_size += tag_size + 24;
							p_manifest.resize(manifest_cur_size);
							
							encode_uint16(0x102, &p_manifest.write[ofs]); 
							encode_uint16(16, &p_manifest.write[ofs + 2]); 
							encode_uint32(tag_size, &p_manifest.write[ofs + 4]); 
							encode_uint32(0, &p_manifest.write[ofs + 8]); 
							encode_uint32(-1, &p_manifest.write[ofs + 12]); 
							encode_uint32(-1, &p_manifest.write[ofs + 16]); 
							encode_uint32(attr_uses_feature_string, &p_manifest.write[ofs + 20]); 
							encode_uint16(20, &p_manifest.write[ofs + 24]); 
							encode_uint16(20, &p_manifest.write[ofs + 26]); 
							encode_uint16(attr_count, &p_manifest.write[ofs + 28]); 
							encode_uint16(0, &p_manifest.write[ofs + 30]); 
							encode_uint16(0, &p_manifest.write[ofs + 32]); 
							encode_uint16(0, &p_manifest.write[ofs + 34]); 
							
							encode_uint32(ns_android_string, &p_manifest.write[ofs + 36]); 
							encode_uint32(attr_name_string, &p_manifest.write[ofs + 40]); 
							encode_uint32(feature_string, &p_manifest.write[ofs + 44]); 
							encode_uint16(8, &p_manifest.write[ofs + 48]);