[shell] Support zim (a zsh framework)

[savil]

Summary

Zim is a framework for zsh. It relies on a .zimrc file for its settings.
If this .zimrc file is not present in ZDOTDIR/.zimrc, then users
get some ugly error messages (and zim itself doesn't work).

Devbox generates a temp-dir for the new devbox shell's configuration files
like .zshrc, or .bashrc, or fish.config.

For zsh:

1. the ZDOTDIR is set to this temp dir.
2. we already symlink some other files like .zprofile.
3. In this PR, we also symlink the .zimrc.

Fixes #1426
Note: the task title asserts that the problem is starship. However, the cause
is that zim is not supported. For good measure, in my testing below I have
enabled starship and it works fine after the fix.

How was it tested?

setup:

1. set zsh as the login shell
2. install zim using the curl install command.

BEFORE:
devbox shell output would spout the error messages from #1426

AFTER:
devbox shell starts without any error messages.

[savil]

Current dependencies on/for this PR:

• main
  • PR [shell] Support zim (a zsh framework) #1428 👈

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