add sticky and stretch keywords

[joyously]

The stretch keyword is not mentioned in the working draft of the spec, but it is supported with a prefix by some browsers. Unfortunately, I couldn't see a good way to do a special case with it for -moz-available.

For part of #6130

[LeaVerou]

Hello, thanks for submitting a PR!

We don't add non-standard things to -prefix-free, the whole point is that if it's non-standard, it should have a prefix in the code to make that fact obvious.

I would be delighted to merge this PR if you only leave sticky in it.

[joyously]

Well, you said in the issue that you would consider it.