Fix bug with extract variable top level statements

[giacomocavalieri]

Experimenting with the nightly compiler I noticed a couple of bugs with the "extract variable" code action I've recently added:

• It would allow extracting top level expressions, not all that useful:

  pub fn main() {
    1
  //^ [Extract variable]
  }
  
  // turned into:
  
  pub fn main() {
    let value = 1
    value
  }

• It would extract module selects on their own instead of considering those as part of an outer function call, resulting in an error:

  pub fn main() {
    list.map(list.filter([1, 2, 3], todo), todo)
  //          ^ [Extract variable]
  }
  
  // turned into:
  
  pub fn main() {
    let value = list.
    list.map(valuefilter([1, 2, 3], todo), todo)
  }

This PR fixes both

[lpil]

Thank you!!!