Add `modify` function to observables` about tko

[avickers]

This brings something else to mind for me: validation functions. Right now there are plugins for knockout to do this for various form elements, but it would be kind of nice to have a first class sanity checking that prevents bad updates whether they come from either user input or corrupted model data being loaded, and then triggers some event/throws some error that can be handled according to context.

[brianmhunt]

@avickers Yes, I've built that for personal work and I've some thoughts about how it could be done more generally — since we're moving into ko 4 beta, I feel like would be appropriate for ko 4.1, alongside a proper first-class router in the default/reference build.

[caseyWebb]

I think modify or mutate would be clear

[mbest]

I suggest that peek be the default.

[brianmhunt]

@mbest Thanks, I was thinking that too.