@block entity (see #1251)

- [x] We should allow `@block{` (no space between `@block` and curly brace)
- [x] We should allow assign for braceless blocks:

    ```
a =
      .bar
        color: red
        if true
          width: 200px
```
- [x] We should allow `@block` as an argument for mixins and in similar places:

    ```
bar(b, c)
      .foo
        {b}
      .bar
        {c}
.baz {
      bar(@block{
        .raz {
          width: 20px
        }
      }, @block{
        height: 20px
      })
}
```