warn on template references to global variables (#2295)

src/compile/Component.ts

@@ -1182,7 +1182,7 @@ export default class Component {
 			if (name[0] === '$') return; // $$props
 		}
 
-		if (this.var_lookup.has(name)) return;
+		if (this.var_lookup.has(name) && !this.var_lookup.get(name).global) return;
 		if (template_scope && template_scope.names.has(name)) return;
 		if (globals.has(name)) return;
 

test/validator/samples/undefined-value-global/input.svelte

@@ -0,0 +1,5 @@
+<script>
+	console.log(potato);
+</script>
+
+<p>{potato}</p>

test/validator/samples/undefined-value-global/warnings.json

@@ -0,0 +1,15 @@
+[{
+	"code": "missing-declaration",
+	"message": "'potato' is not defined",
+	"pos": 46,
+	"start": {
+		"line": 5,
+		"column": 4,
+		"character": 46
+	},
+	"end": {
+		"line": 5,
+		"column": 10,
+		"character": 52
+	}
+}]