fix(electric,elixir-client): Write static version string to mix.exs prior to publishing

[alco]

Fixes #2179

[icehaunter]

I'm not against this approach, but in elixir-client it's solved just by including package.json in published filies. Maybe it's easier to just do that here too?

[magnetised]

I solved this for the client but forgot about the top-level package. Including the package.json in the package files is a much simpler and more robust fix -- I deliberately avoided doing any source-code munging because it scares me.

if we just add files: to the package() output and include package.json there it works perfectly:

  defp package do
    [
      licenses: ["Apache-2.0"],
      links: %{
        "Electric SQL" => "https://electric-sql.com",
        "Github" => @github_repo
      },
      files: ~w(lib .formatter.exs mix.exs README.md LICENSE package.json)
    ]
  end

[magnetised]

see my and @icehaunter's comments re package.json in package files -- feels like a more robust solution

[magnetised]

@alco happy to finish this since I feel responsible ... 😊