Expand values into arrays as needed when setting with nestedProperty

[rreusser]

See #1580

This is just speculative. I don't necessarily think it's a good idea, but I'm submitting it to confirm whether it's actually a bad idea. If the current value is not an array or an object and you try to set an array value, it wraps the current value in an array before setting. Unless I'm mistaken, this should only affect cases that currently throw an error.

[rreusser]

The best options I can come up with:

1. this PR, except maybe fill in all values up to the requested index with the current value.
2. restyle/relayout/nestedproperty need more knowledge about the schema so they can enable this only for arrayOk attributes
3. we should disallow non-data attributes that fall into this pattern (setting data attributes with an accessor like this isn't an interesting use case anyway, but nestedProperty results in modal Plotly.restyle #1580 highlights why the difficulty is problematic for non-data attrs that need to get set in the ui)
4. work around the problems by digging into fullData in client code

[rreusser]

Closing this as a discussion point but probably not the right solution.