add: rnix-lsp overlays

Add `rnix-lsp` `overlays` to the flake outputs, in order to be directly
consumed by other flakes.

Provides 2 overlays:
- default: with package named `rnix-lsp`, so people can overwrite their
  existing configuration
- nightly: with package named `rnix-lsp-nightly`, so people can opt in
  to this package on a case-by-case basis

Also for now still uses the `overlay` attribute, for backwards
compatibility, is that needed?

fixes: nix-community#77

flake-compat.nix

@@ -3,7 +3,6 @@ let
     url = "https://github.com/edolstra/flake-compat/archive/12c64ca55c1014cdc1b16ed5a804aa8576601ff2.tar.gz";
     sha256 = "0jm6nzb83wa6ai17ly9fzpqc40wg1viib8klq8lby54agpl213w5";
   };
-in
-(import flake-compat {
+in (import flake-compat {
   src = ./.;
 })

flake.nix

@@ -35,5 +35,17 @@
           drv = packages.rnix-lsp;
         };
         defaultApp = apps.rnix-lsp;
-      });
+      })
+      //
+      rec {
+        overlays = { 
+          default = final: prev: rec {
+          rnix-lsp = self.packages.${prev.system}.rnix-lsp;
+        };
+          nightly = final: prev: rec {
+          rnix-lsp-nightly = self.packages.${prev.system}.rnix-lsp;
+          };
+        };
+        overlay = overlays.default;
+    };
 }