Throw if `decl()` has no `block` field

[Yeti-or]

example of code with such error:

import MenuItem from 'b:menu e:item m:type=link';

export default decl(MenuItem, {
+   block: 'user',
+   elem: 'menu-item',
    mix({action}) {
        return {block: 'user', elem: 'menu-item', mods: {action}};
    },
    attrs() {
        return {
            ...this.__base(...arguments),
            href: this.url,
            tabIndex: '0'
        };
    }
});

[awinogradov]

I proposed this diff as normal declaration. Because we can understand entities from the file context. cc @veged