Case split plugin

The goal of this proposal is to extend HLS with a new plugin, the case split plugin, which provides a new code action (or possibly more of them) for completing the case ⟨expression⟩ of part of a case expression with the properly formatted list of alternatives, which is implied by the type of ⟨expression⟩. GHC API will be used to detect the cases missing from a (totally or partially) incomplete case expression, and to insert the missing cases in the AST, ghc-exactprint will be used for pretty-printing such missing cases in the text file.