feat(lsp)!: remove client-server handlers from vim.lsp.handlers

- Partition the handlers in vim.lsp.handlers as:
  - client to server response handlers (RCS)
  - server to client request handlers (RSC)
  - server to client notification handlers (NSC)

  Note use string indexes instead of protocol.methods for improved
  typing in LuaLS (tip: use hover on RCS, RSC or NSC).

runtime/lua/vim/lsp/codelens.lua

@@ -261,7 +261,7 @@ end
 ---@param err lsp.ResponseError?
 ---@param result lsp.CodeLens[]
 ---@param ctx lsp.HandlerContext
-function M.on_codelens(err, result, ctx, _)
+function M.on_codelens(err, result, ctx)
   if err then
     active_refreshes[assert(ctx.bufnr)] = nil
     log.error('codelens', err)