Add HaskellNet

See merge request horizon/horizon-platform!51

horizon-spec: 0.6 -> 0.6.1

See merge request horizon/horizon-platform!50

horizon-gen-nix: 0.5 -> 0.6

See merge request horizon/horizon-platform!49

horizon-spec: 0.5 -> 0.6

Stabilize some repositories

See merge request horizon/horizon-platform!48

double-conversion: github:haskell/double-conversion/5d092e0664442eaac8ae1d101dba57ce9b1c9b03 -> 2.0.4.2

tasty-hedgehog: github:locallycompact/tasty-hedgehog/4a3477578ed21aa82e5b74f387d08e5d750635d6 -> 1.4.0.0

Add haskell-language-server

See merge request horizon/horizon-platform!47

Introduced with dependencies it needs, with a number of plugins disabled
that don't build with ghc-9.4.x yet.

algebraic-graphs downgraded to 0.6.1 due to incompatibility in hiedb.

This is more reliable than trying to use the previous horizon-platform
version.