Add a lint for using @each with maps properly

[nex3]

A pattern I see occasionally is:

@each $key in map-keys($map) {
  $value: map-get($map, $key);
  // ...
}

This is an unnecessarily verbose way of writing:

@each $key, $value in $map {
  // ...
}

There are also some legitimate cases where only the key is used, so maybe it's a good idea to look for the pattern map-get($map, $key) in the loop body before raising the lint?