tests: validator: add tests for tag usage with/without customElement option

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

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

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

@@ -0,0 +1,15 @@
+[{
+    "code": "missing-custom-element-compile-options",
+    "message": "The 'tag' option is used when generating a custom element. Did you forget the 'customElement: true' compile option?",
+    "start": {
+        "line": 1,
+        "column": 16,
+        "character": 16
+    },
+    "end": {
+        "line": 1,
+        "column": 36,
+        "character": 36
+    },
+    "pos": 16
+}]

test/validator/samples/tag-custom-element-options-true/_config.js

@@ -0,0 +1,3 @@
+export default {
+    customElement: true,
+};

test/validator/samples/tag-custom-element-options-true/input.svelte

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