### 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(attr_uses_feature_string, &p_manifest.write[ofs + 20]); 
							ofs += 24;
						}
						memcpy(&p_manifest.write[ofs], manifest_end.ptr(), manifest_end.size());
						ofs -= 24; 
					}
				}
				if (tname == "manifest") {
					
					Vector<uint8_t> manifest_end;
					uint32_t manifest_cur_size = p_manifest.size();
					manifest_end.resize(p_manifest.size() - ofs);
					memcpy(manifest_end.ptrw(), &p_manifest[ofs], manifest_end.size());
					int32_t attr_name_string = string_table.find("name");
					ERR_FAIL_COND_MSG(attr_name_string == -1, "Template does not have 'name' attribute.");
					int32_t ns_android_string = string_table.find("android");
					ERR_FAIL_COND_MSG(ns_android_string == -1, "Template does not have 'android' namespace.");
					int32_t attr_uses_permission_string = string_table.find("uses-permission");
					if (attr_uses_permission_string == -1) {
						string_table.push_back("uses-permission");
						attr_uses_permission_string = string_table.size() - 1;
					}
					for (int i = 0; i < perms.size(); ++i) {
						print_line("Adding permission " + perms[i]);
						manifest_cur_size += 56 + 24; 
						p_manifest.resize(manifest_cur_size);
						
						int32_t perm_string = string_table.find(perms[i]);
						if (perm_string == -1) {
							string_table.push_back(perms[i]);