plugins/asdf: Add `asdf` extendable version manager plugin

[RobLoach]

This change adds a plugin to load the asdf version manager through oh-my-bash. It's roughly based off of the original pull request, moves to the bash syntax, and allows checking for the ASDF_DIR variable before hitting the file system.

[RobLoach]

Thanks a lot for the review, I'll fix these up.

[RobLoach]

I didn't consider the commit message format here, so when merging, feel free to squash on merge. Thanks a lot for the reviews.

[akinomyoga]

Thank you.

[RobLoach]

Thank you so much for the reviews and merge :+1: ... Happy to have this in.