### 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
String err;
	bool valid = false;
	const bool custom_build_enabled = p_preset->get("custom_build/use_custom_build");
	
	if (!custom_build_enabled) {
		String template_err;
		bool dvalid = false;
		bool rvalid = false;
		bool has_export_templates = false;
		if (p_preset->get("custom_template/debug") != "") {
			dvalid = FileAccess::exists(p_preset->get("custom_template/debug"));
			if (!dvalid) {
				template_err += TTR("Custom debug template not found.") + "\n";
			}
		} else {
			has_export_templates |= exists_export_template("android_debug.apk", &template_err);
		}
		if (p_preset->get("custom_template/release") != "") {
			rvalid = FileAccess::exists(p_preset->get("custom_template/release"));
			if (!rvalid) {
				template_err += TTR("Custom release template not found.") + "\n";
			}
		} else {
			has_export_templates |= exists_export_template("android_release.apk", &template_err);
		}