Fix non-probing of union array failure

Probing a union match for an possible value should rarely hard-fail. Instead,
it should return an .{.invalid = {}} response to let the prober decide how to
proceed. This fixes a hard-fail when a JS value fails to probe as an array.

Also, add :modal pseudo-class.

(both issues came up looking at github integration)

src/runtime/js.zig

@@ -1158,7 +1158,7 @@ pub fn Env(comptime State: type, comptime WebApis: type) type {
                             }
 
                             if (!js_value.isArray()) {
-                                return error.InvalidArgument;
+                                return .{.invalid = {}};
                             }
 
                             // This can get tricky.