lsp: add support for several LSP clients (#111)

* nvim-lsp: Allow to pass settings to clients

Some clients (like rust-analyzer, nil_ls, ...) can take settings
specified in the setup function. This commit adds two fields for the
'mkLsp' function to handle this:

- extraOptions: define nix module options
- settings: A function that takes the corresponding lsp module config
  and formats it as a settings object compatible with the server.

* nvim-lsp: Add nil-ls (for Nix)

* nvim-lsp: Add bashls (for Bash)

* nvim-lsp: Add dartls, for dart

Co-authored-by: Pedro Alves <pta2002@users.noreply.github.com>

plugins/nvim-lsp/helpers.nix

@@ -7,6 +7,8 @@
     , serverName ? name
     , packages ? [ pkgs.${name} ]
     , cmd ? null
+    , settings ? null
+    , extraOptions ? { }
     , ...
     }:
     # returns a module
@@ -19,7 +21,7 @@
         options = {
           plugins.lsp.servers.${name} = {
             enable = mkEnableOption description;
-          };
+          } // extraOptions;
         };
 
         config = mkIf cfg.enable {
@@ -29,6 +31,7 @@
             name = serverName;
             extraOptions = {
               inherit cmd;
+              settings = if settings != null then settings cfg else { };
             };
           }];
         };