Enhance simple eval plugin to allow evaluating within scope of functions

[codygman]

Given this:

main = do
    let name = "codygman"
   -- >>> name
    in pure ()

I'd want the evaluation of name to work resulting in:

main = do
    let name = "codygman"
   -- >>> name
   -- codygman
    in pure ()

This would require bringing all bindings up to that point into scope. Typically I copy paste definitions in ghci like:

> let name = "codygman"
> name

I can see how this could be complex and have some edge cases, but the value of it would be immense especially for new Haskell users.

In fact this request comes because I was making an intro video "0 to Haskell ide in 5 minutes" type video and this feature was going to be the sort of centerpoint/ending to that video.

This would allow not really needing to use anything but haskell-language-server for development as well instead of having to open ghci for many cases. Regardless of whether that's always better, I think it's a good option to have.

I think this might be blocked or more difficult to implement if the current implementation isn't using -fexternal-interpreter as described here.

related: #94 #191 #51

[konn]

I find this feature really useful too! It could eliminate the infamous uses of trace function in many cases.

On the other hand, I think there are several corner cases to be considered.

For example, consider the following:

add n m =  do
  let k = n + m
  -- >>> m
  pure k

Then, what the result of >>> m must be?
That is, how could we define the "evaluation result" containing bound variables?