Mark unknown functions

[johakoch]

Whereas unknown blocks and attributes are marked with a red wiggly line and have a tooltip "Unknown ...", unknown functions are not marked in any way.

[johakoch]

Unknown functions are actually marked as "invalid" if they are used as attribute values, like in

request {
  url = foo()
}

However, if they are used e.g. as values in an object like in

response {
  json_body = {
    a = foo()
  }
}

they are not marked.