### 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
static const char *GDEXTENSION_LIBS_PATH = "res://android/build/libs/gdextensionlibs.json";
static const int icon_densities_count = 6;
static const char *launcher_icon_option = PNAME("launcher_icons/main_192x192");
static const char *launcher_adaptive_icon_foreground_option = PNAME("launcher_icons/adaptive_foreground_432x432");
static const char *launcher_adaptive_icon_background_option = PNAME("launcher_icons/adaptive_background_432x432");
static const LauncherIcon launcher_icons[icon_densities_count] = {
	{ "res/mipmap-xxxhdpi-v4/icon.png", 192 },
	{ "res/mipmap-xxhdpi-v4/icon.png", 144 },
	{ "res/mipmap-xhdpi-v4/icon.png", 96 },
	{ "res/mipmap-hdpi-v4/icon.png", 72 },
	{ "res/mipmap-mdpi-v4/icon.png", 48 },
	{ "res/mipmap/icon.png", 192 }
};
static const LauncherIcon launcher_adaptive_icon_foregrounds[icon_densities_count] = {
	{ "res/mipmap-xxxhdpi-v4/icon_foreground.png", 432 },
	{ "res/mipmap-xxhdpi-v4/icon_foreground.png", 324 },
	{ "res/mipmap-xhdpi-v4/icon_foreground.png", 216 },
	{ "res/mipmap-hdpi-v4/icon_foreground.png", 162 },
	{ "res/mipmap-mdpi-v4/icon_foreground.png", 108 },
	{ "res/mipmap/icon_foreground.png", 432 }
};
static const LauncherIcon launcher_adaptive_icon_backgrounds[icon_densities_count] = {
	{ "res/mipmap-xxxhdpi-v4/icon_background.png", 432 },
	{ "res/mipmap-xxhdpi-v4/icon_background.png", 324 },
	{ "res/mipmap-xhdpi-v4/icon_background.png", 216 },
	{ "res/mipmap-hdpi-v4/icon_background.png", 162 },
	{ "res/mipmap-mdpi-v4/icon_background.png", 108 },
	{ "res/mipmap/icon_background.png", 432 }
};
static const int EXPORT_FORMAT_APK = 0;