als deprecation warning

[dundalek]

Getting a warning message on nvim startup with recently updated lspconfig (deprecating als neovim/nvim-lspconfig#3277):

It is strange since we don't have a corresponding nix package for als. Maybe we need to guard it in additional condition and not try to even access server config when we don't have a package (and when there is no user provided configuration)?

As a workaround for now one can put als in excluded_servers option.

[dundalek]

The als issue got addressed in #49 so this issue is about making things more resilient against future changes/renames.