Case insensitive sort_keys

[DAVe3283]

Please describe your feature request.
Ability to sort keys ignoring case (not case sensitive).

Describe the solution you'd like
If we have data1.yml like:

Foo: 3
apple: 1
bar: 2

And we run a command:

yq -P 'sort_keys_insensitive(..)' data1.yml

it could output

apple: 1
bar: 2
Foo: 3

Describe alternatives you've considered
I tried to use 'sort_keys(..|downcase)' but it does not work (probably as expected).

Additional context
While I don't want to consider case for the purpose of the sort, I would like to retain the case of the keys in the output.

[mikefarah]

Will accept a PR for this, this should be done by adding a sort_keys_by function (just as there is a sort_by function here)

You will then be able to: sort_keys_by(downcase)

[mikefarah]

Otherwise I may get to it in a few weeks :)