warn if using svelte:options tag without compile_options.customElement

src/compiler/compile/Component.ts

@@ -1307,6 +1307,13 @@ function process_component_options(component: Component, nodes) {
 							});
 						}
 
+						if (tag && !component.compile_options.customElement) {
+							component.warn(attribute, {
+								code: 'missing-custom-element-compile-options',
+								message: `tag name is used when compiling the compenent as a custom element. Did you forgot to add "customElement" for compile options?`
+							});
+						}
+
 						component_options.tag = tag;
 						break;
 					}

test/validator/samples/missing-custom-element-compile-options/input.svelte

@@ -0,0 +1 @@
+<svelte:options tag="custom-element" />

test/validator/samples/missing-custom-element-compile-options/warnings.json

@@ -0,0 +1,17 @@
+[
+	{
+		"code": "missing-custom-element-compile-options",
+		"end": {
+			"character": 36,
+			"column": 36,
+			"line": 1
+		},
+		"message": "tag name is used when compiling the compenent as a custom element. Did you forgot to add \"customElement\" for compile options?",
+		"pos": 16,
+		"start": {
+			"character": 16,
+			"column": 16,
+			"line": 1
+		}
+	}
+]