Add test for non-writable binding check

test/validator/samples/binding-const/errors.json

@@ -0,0 +1,15 @@
+[{
+	"code": "invalid-binding",
+	"message": "Cannot bind to a variable which is not writable",
+	"pos": 61,
+	"start": {
+		"line": 5,
+		"column": 19,
+		"character": 61
+	},
+	"end": {
+		"line": 5,
+		"column": 24,
+		"character": 66
+	}
+}]

test/validator/samples/binding-const/input.svelte

@@ -0,0 +1,5 @@
+<script>
+	const dummy = 'foo';
+</script>
+
+<input bind:value={dummy}>