Command: Evaluate at position

[julienduchesne]

The language server currently has the evaluateFile and evaluateExpression commands. Another very useful one, but much harder to implement, is evaluateAtPosition. This could allow a user to check the value of self, or a local at any location

Since locals cannot be queried, we could define temporary attributes at runtime, like this:

{
  local my_key = 'test',
  temp:: my_key, // Line added at eval time (not defined by the user)
}