Fix: preserve datetime precision after Timex.shift/2

[emancu]

Resolves #731

Unfortunately this broke with elixir-lang/elixir@5a583c7 which was release with Elixir 1.14.3

Elixir 1.13.4:

iex(3)> DateTime.utc_now() |> IO.inspect() |> DateTime.truncate(:second) |> IO.inspect() |> Timex.shift(minutes: 1) |> IO.inspect()
~U[2023-04-13 09:56:10.136274Z]
~U[2023-04-13 09:56:10Z]
~U[2023-04-13 09:57:10Z]

Elixir 1.14.4:

iex(1)> DateTime.utc_now() |> IO.inspect() |> DateTime.truncate(:second) |> IO.inspect() |> Timex.shift(minutes: 1) |> IO.inspect()
~U[2023-04-13 09:55:16.405357Z]
~U[2023-04-13 09:55:16Z]
~U[2023-04-13 09:56:16.000000Z]

Summary of changes

I'll review the commits, so I mostly want to understand the "why" rather than the "what"

Checklist

• New functions have typespecs, changed functions were updated
• Same for documentation, including moduledocs
• Tests were added or updated to cover changes
• Commits were squashed into a single coherent commit
• Notes added to CHANGELOG file which describe changes at a high-level