v.gen.js: Interfaces support, fix for error propagation & panic stacktraces (#11471)

vlib/builtin/js/builtin.js.v

@@ -79,3 +79,12 @@ pub fn (r rune) str() string {
 
 	return sb.str()
 }
+
+fn js_stacktrace() string {
+	stacktrace := ''
+	#let err = new TypeError();
+	#err.name = 'stacktrace: '
+	#stacktrace.str = err.stack
+
+	return stacktrace
+}

vlib/builtin/js/builtin.v

@@ -7,7 +7,7 @@ module builtin
 fn (a any) toString()
 
 pub fn panic(s string) {
-	eprintln('V panic: $s')
+	eprintln('V panic: $s\n$js_stacktrace()')
 	exit(1)
 }