Expand completion support for language server

[jabraham17]

This PR expands upon our existing completion support to be location specific. Prior to this PR, only the module level scope was used to provide completion symbols. This PR changes that, so that visible symbols are computed based upon the users cursor.

With this PR, users will get an auto-complete suggestion of 'myVariable' when 'myV' is typed.

record myRecord {
  var  myVariable = 1;
  proc foo() {
    myV
  }
}

Implementation details

This PR uses the fact that the FileInfo visits nodes in order when building segments. So in the above example, the scope for myRecord is visited before foo. As each scope is entered, we keep track of the location, its Scope object, and the parent scopes. This is precomputed before a user ever requests a completion, so that when that occurs it is just a simple lookup.

[Reviewed by @DanilaFe]

[DanilaFe]

Some preliminary comments