[feat] websockets (#620)

Allow the user to set up an lsp server that talks to the user using
the websocket protocol instead of standard IO and do enough refactoring
to make the setup with websockets possible

Whenever a new message arrives on the websocket, push the new message as
well as an empty message as the followup - additionally, do not throw an
error when the parser encounters an empty ByteString, it should be able
to handle that by itself.

Link the lspAsync to cascade a dead server to its sockets

Author: MangoIV

lsp/lsp.cabal

@@ -79,6 +79,7 @@ library
     , unliftio              ^>=0.2
     , unliftio-core         ^>=0.2
     , unordered-containers  ^>=0.2
+    , websockets            ^>=0.13
 
 executable lsp-demo-reactor-server
   import:             warnings