Allow init --skip-env and --only-env

[calebhearth]

In (some?) shells, it's sometimes preferable to set up env vars in a
different place than other configuration like most of what atuin init
sets up. For example, ZSH reads ~/.zshenv for all shells while ~/.zshrc
is only read for interactive shells.

This allows splitting the output of init such that in these two
different config files we could load environment variables and load all
other config seperately:

~/.zshenv:

eval "$(atuin init zsh --env-only)"

~/.zshrc:

eval "$(atuin init zsh --skip-env --disable-up-arrow)"

Which means that when launching noninteractive shells (I do this
frequently for running tests from Vim, for example, but it's more
generally useful) the specified env values will be sourced and we won't
redefine them in login shells.

Checks

• I am happy for maintainers to push small adjustments to this PR,
  to speed up the review cycle
• I have checked that there are no existing pull requests for the same thing

Note: I know I didn't create an issue or post anywhere else first. If
this is totally not a desired feature my feelings won't be hurt if you
say so and close this.

[calebhearth]

I took a look at the clippy errors. I was able to fix a docs suggestion but extracting an enum and sharing across files proved more complicated. I've not done much rust before and while I had no problem (and my initial instinct had been to) extracting a 3 value enum, actually sharing that across files exceeded my skill level.

[calebhearth]

@ellie I know you're busy with Desktop, but wondering if you have a bit to look at this. I'm happy to try and fix clippy if you want to point me in the right direction, or if you know how to do the fix easily feel free.

[cosgroveb]

<3 this is a worthwhile enhancement

[cosgroveb]

CONTIRUBUTING.md doesn't say it but maybe @ellie would be more responsive with a corresponding issue?