### 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 (string_flags & UTF8_FLAG) {
					} else {
						uint32_t len = decode_uint16(&p_manifest[string_at]);
						Vector<char32_t> ucstring;
						ucstring.resize(len + 1);
						for (uint32_t j = 0; j < len; j++) {
							uint16_t c = decode_uint16(&p_manifest[string_at + 2 + 2 * j]);
							ucstring.write[j] = c;
						}
						string_end = MAX(string_at + 2 + 2 * len, string_end);
						ucstring.write[len] = 0;
						string_table.write[i] = ucstring.ptr();
					}
				}
				for (uint32_t i = string_end; i < (ofs + size); i++) {
					stable_extra.push_back(p_manifest[i]);
				}
				string_table_ends = ofs + size;
			} break;
			case CHUNK_XML_START_TAG: {
				int iofs = ofs + 8;
				uint32_t name = decode_uint32(&p_manifest[iofs + 12]);
				String tname = string_table[name];
				uint32_t attrcount = decode_uint32(&p_manifest[iofs + 20]);
				iofs += 28;
				for (uint32_t i = 0; i < attrcount; i++) {
					uint32_t attr_nspace = decode_uint32(&p_manifest[iofs]);
					uint32_t attr_name = decode_uint32(&p_manifest[iofs + 4]);
					uint32_t attr_value = decode_uint32(&p_manifest[iofs + 8]);
					uint32_t attr_resid = decode_uint32(&p_manifest[iofs + 16]);