Command: Evaluate at position

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)
}
```

Start by reading the existing evaluateFile and evaluateExpression language-server commands and determine how an evaluateAtPosition request could access self or locals. Done means the command can evaluate the value at a requested position, including a local variable, with the runtime approach resolved and tested.

Written by the indexing model from the issue text.