add toggle action to invokers dialog

[keithamus]

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When prototyping/implementing the behaviour for invokers on <dialog>, I noticed that toggle wasn't an action available, only toggleModal.

For completeness and compatibility with non-modal dialogs, I went ahead and implemented toggle, which begs the question: should it exist?

Rather than raising an issue I thought I'd raise a PR for us to speculate on.

One reason to not add this is https://github.com/whatwg/html/issues/9376 - which asks if we should deprecate .show() altogether.

/cc @lukewarlow @mfreed7 @scottaohara

[lukewarlow]

I think it should exist for completeness. Unless we make it so invokers ONLY work for modal dialogs? Which could be an option?

Also gonna link #920

[lukewarlow]

Linking #935 discussion