Support nodejs >= 15, npm >= 7, lockfile version >= 2

[infinisil]

nodejs versions >= 15 use npm >= 7, which uses lockfile version 2 which is currently not supported by npmlock2nix, leading to failures like #139 and #140. See #146 for an attempt at implementing it. In addition, hook scripts, which npmlock2nix currently relies on, are also removed, see #110 and also #151 as a workaround.

PR #152 makes npmlock2nix fail early for nodejs version >= 15.