Add optionalExtensions attribute to better handle missing nightly extensions

[evanjs]

Motivation for this change

Add several functions to aid in the resolution of potentially missing nightly extensions

• Add optionalExtensions attribute
• Add getOptionalExtensions function
• Add getOptionalAndRequiredExtensions function

I wrote these up a while ago as a potential solution to #108 as mentioned here.

It's been a while since I looked at this, but I wanted to open this in order to gather feedback on potential solutions to this issue, be it work to get this PR working with what's currently in tree, or a different approach altogether.

I would love to see some of the features in rustup make their way here, but am uncertain of the best approach.

I have noted several options in the rust module in my configuration, but am curious what others think.

I think there are several questions need to be answered in order to resolve this. However, I want to first verify whether this is something that should even be handled by this repository.

At the very least, it might be beneficial to provide such an option as an alternative resolution method for nightly extensions for those that wish to opt in to such behavior.

[evanjs]

Recently discovered the Oxalica overlay which seems to implement at least one of the aforementioned approaches (latest nightly with specified extensions) as mentioned here.