feat: preserve externally passed list types

[nikku]

Which issue does this PR address?

On top of #70 this PR ensures that we recognize types from externally passed lists:

# List (union type, external) { "context": { "list": [ { "a+": 1 }, { "b+": 1 } ] } }

list.b+

==>

Expression(
  PathExpression(VariableName(...),VariableName(...))
)

Today, those are not recognized.

---

Draft, because the alternative is that we improve typing of filter expressions list[1], and keep existing shape of externally passed list as an object with named (1, 2, ...) properties.