### 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
p_manifest.write[ofs + 50] = 0; 
							p_manifest.write[ofs + 51] = 0x03; 
							encode_uint32(feature_string, &p_manifest.write[ofs + 52]); 
							
							encode_uint32(ns_android_string, &p_manifest.write[ofs + 56]); 
							encode_uint32(attr_required_string, &p_manifest.write[ofs + 60]); 
							encode_uint32(required_value, &p_manifest.write[ofs + 64]); 
							encode_uint16(8, &p_manifest.write[ofs + 68]); 
							p_manifest.write[ofs + 70] = 0; 
							p_manifest.write[ofs + 71] = 0x03; 
							encode_uint32(required_value, &p_manifest.write[ofs + 72]); 
							ofs += 76;
							if (has_version_attribute) {
								
								encode_uint32(ns_android_string, &p_manifest.write[ofs]); 
								encode_uint32(attr_version_string, &p_manifest.write[ofs + 4]); 
								encode_uint32(version_value, &p_manifest.write[ofs + 8]); 
								encode_uint16(8, &p_manifest.write[ofs + 12]); 
								p_manifest.write[ofs + 14] = 0; 
								p_manifest.write[ofs + 15] = 0x03; 
								encode_uint32(version_value, &p_manifest.write[ofs + 16]); 
								ofs += 20;
							}
							
							encode_uint16(0x103, &p_manifest.write[ofs]); 
							encode_uint16(16, &p_manifest.write[ofs + 2]); 
							encode_uint32(24, &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]);