Support pnpm lockfileVersion: 6.0

[oatmealine]

PNPM has introduced a new lockfile format, which seems to be entirely incompatible with this tool:

error: assertion '(((pnpmlock).lockfileVersion == 5) || ((pnpmlock).lockfileVersion == 5.1))' failed

       at /nix/store/95yf6y552q2xvh5f39ma8kvb5iiaj7m9-source/default.nix:201:5:

          200|   in
          201|     assert (pnpmlock.lockfileVersion == 5 || pnpmlock.lockfileVersion == 5.1);
             |     ^
          202|   (mkPnpmDerivation {

I'm not sure if this is still maintained (#23) - but I'd very much appreciate an update to this