Add support for contenteditable=true in iterateFocusableElements

[JelloBagel]

Elements with contenteditable=true are focusable natively in the browser. We would like for iterateFocusableElements method to also account for contenteditable when checking for focusable elements

MDN: https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/contenteditable

Snippet in iterate-focusable-elements.ts where focus is checked: https://github.com/primer/behaviors/blob/main/src/utils/iterate-focusable-elements.ts#L86

[keithamus]

Thanks for the issue @JelloBagel! Would you be interested in making a PR for this?

[JelloBagel]

@keithamus sure, I can take a stab at it when I get a moment this week or the next 😃