feat: add atuin doctor

[ellie]

This should help a bunch with diagnosing support issues

I've built it to output a datastructure as YAML. The reason here is that in the future, we can easily build functions that will take setup information and suggest fixes for common problems.

Or even automatically fix them!

Plugin detection right now has a couple notes

1. It only detects bash preexec or ble. This is most of what we are concerned about at the moment
2. It relies on running env in a shell subprocess. Many env vars that are useful to detect plugins are not exported. This is the only easy way I can see right now, though it is kinda gross.

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