jsgen: fix casting (fix #24512) (#24519)

vlib/v/gen/js/js.v

@@ -872,8 +872,8 @@ fn (mut g JsGen) expr(node_ ast.Expr) {
 			g.gen_array_init_expr(node)
 		}
 		ast.AsCast {
-			// skip: JS has no types, so no need to cast
 			// TODO: Is jsdoc needed here for TS support?
+			g.expr(node.expr)
 		}
 		ast.Assoc {
 			// TODO

vlib/v/gen/js/tests/casting.v

@@ -0,0 +1,6 @@
+type Type = string | bool
+
+fn main() {
+	t := Type('')
+	assert t is string && t == ''
+}