Style grade nested+empty let expressions/local declarations

[T-Brick]

Issue

Currently, we allow empty let expressions (ones with no declarations), as well as nested let expressions (similarly for local declarations). For instance,

val _ =
    let
    in
        150
    end

val _ =
    let
        val x = 50
    in
        let
            val y = 100
        in
            x + y
        end
    end

Is there any reason we should allow this? Should we even bother creating rules for these? I know anecdotally some students have said that they didn't know you could put multiple declarations in a let expression, maybe having a style rule like that would help?

Next Steps

λdditional Info

• Relevant Semester: F21

Successor issue to https://github.com/15-150/15150/issues/1744