Enable js macro in more context

Once in a lifetime, situation does not allow for `let` keyword in certain contexts (example below, not the best but still proves the point). There is no apparent downside to this extra wrap in `{}`. Alternatively, `let` can be safely remove instead of this.

This seemed so minor of a change that I did not bother with testing, reading any docs. I just saw no comments around the macro so I assumed it's safe to modify and write about. Sorry!

```rust
error: expected expression, found `let` statement
   --> src/x.rs:185:23
    |
185 |     this1.and_then(|i| js!(o["this1"] = i));
    |                       ^^^^^^^^^^^^^^^^^^
    |
    = note: only supported directly in conditions of `if` and `while` expressions
    = note: this error originates in the macro `js` (in Nightly builds, run with -Z macro-backtrace for more info)
```

Author: Davoodeh

src/utils/js.rs

@@ -4,7 +4,9 @@ macro_rules! js {
         wasm_bindgen::JsValue::from($attr).js_in($($obj)*)
     };
     ($obj:ident[$attr:literal] = $($val:tt)*) => {
-        let _ = js_sys::Reflect::set(&$obj, &$attr.into(), &($($val)*).into());
+        {
+            let _ = js_sys::Reflect::set(&$obj, &$attr.into(), &($($val)*).into());
+        }
     };
     ($obj:ident[$attr:literal]) => {
         js_sys::Reflect::get(&$obj, &$attr.into())
@@ -16,5 +18,4 @@ macro_rules! js_fut {
     ($($obj:tt)*) => {
         wasm_bindgen_futures::JsFuture::from($($obj)*)
     };
-
 }