add sigil-expression-function-body test against regression (sveltejs#3756)

Author: Conduitry

test/runtime/samples/sigil-expression-function-body/_config.js

@@ -0,0 +1,3 @@
+export default {
+	html: `@foo`
+};

test/runtime/samples/sigil-expression-function-body/main.svelte

@@ -0,0 +1 @@
+{(() => '@foo')()}