Add safe Nix integer warning or compile error

[PgBiel]

Similarly to how Gleam now warns on unsafe integers on the JS target (see gleam-lang/gleam#3764), we could do the same for Nix since Nix restricts integer literals to between -9223372036854775807 and 9223372036854775808 (signed 64-bit integer values, though to get -9223372036854775808 it is necessary to write -9223372036854775807 - 1).

builtins.fromJSON for higher literals will simply convert to float, whereas, without fromJSON, the literals just cause a parsing error (even if they're not evaluated). It could even straight up become a compilation error.