Rewrite atom interpolations - non keywords

[sabiwara]

Since we are soft-deprecating the :"foo#{x}" in favor of String.to_unsafe_atom/1, I figured I'd rewrite these in the codebase already.

I added a new migrate_atom_interpolations formatter option to perform this (first commit).
We don't have to keep it though if we don't want to, I wanted this for myself and figured it might help people address this deprecation more easily (which is the point of --migrate).

If we want to indeed support the option, will add proper tests and support for keywords as well, I wanted to get feedback first.

[josevalim]

Looks good to me. My only concern is that the migrate_atom_interpolation will push everyone to Elixir v1.21 but given migration is opt-in, we are likely fine!