Swap `sortKeys` bool for `keySort` function

[kevboh]

(This is a breaking change, but I can make it non-breaking if preferred. It's just a lot more code repetition if non-breaking.)

This changes the sortKeys option to be a more flexible keySort function. The function defaults to nil—do not sort—but can be anything that sorts Nodes. I ran into an issue where I wanted a custom sort on a mapping.

[jpsim]

Thanks for the PR! Could you please rebase your commits on top of main? That should fix the failing CMake CI jobs.

[kevboh]

@jpsim this has been rebased. Thanks for taking a look!

[kevboh]

@jpsim let me know if anything else is required here.

[kevboh]

Hi, is this still mergeable? If so I'd love to merge it!