### 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
"CAMERA",
	"CAPTURE_AUDIO_OUTPUT",
	"CAPTURE_SECURE_VIDEO_OUTPUT",
	"CAPTURE_VIDEO_OUTPUT",
	"CHANGE_COMPONENT_ENABLED_STATE",
	"CHANGE_CONFIGURATION",
	"CHANGE_NETWORK_STATE",
	"CHANGE_WIFI_MULTICAST_STATE",
	"CHANGE_WIFI_STATE",
	"CLEAR_APP_CACHE",
	"CLEAR_APP_USER_DATA",
	"CONTROL_LOCATION_UPDATES",
	"DELETE_CACHE_FILES",
	"DELETE_PACKAGES",
	"DEVICE_POWER",
	"DIAGNOSTIC",
	"DISABLE_KEYGUARD",
	"DUMP",
	"EXPAND_STATUS_BAR",
	"FACTORY_TEST",
	"FLASHLIGHT",
	"FORCE_BACK",
	"GET_ACCOUNTS",
	"GET_PACKAGE_SIZE",
	"GET_TASKS",
	"GET_TOP_ACTIVITY_INFO",
	"GLOBAL_SEARCH",
	"HARDWARE_TEST",
	"INJECT_EVENTS",
	"INSTALL_LOCATION_PROVIDER",