Installation of Elixir by the build tool

[lpil]

Wojtek suggested that the Gleam build tool could install Elixir for the user if needed: https://github.com/elixir-mint/mint/issues/414#issuecomment-1814410314

This could be a nice DX improvement.

Considerations

How do we make it clear that such a large dependency is being included? I think we don't want to have folks accidentally add a very large amount of code to their application when they add a small dependency.