Add easy button to enable "prefers-reduced-motion" in debug window

[stefanh]

In the debug dev tools inspector window, there are buttons to preview how the website looks with certain "prefers-color-scheme" settings enabled. I propose the addition of a button to preview the site with "prefers-reduced-motion" as toggling this setting on and off in firefox's "about:config" page isn't immediately obvious and not as fast.

[Jon]

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[Honza]

Yes, agreed, this would be nice addition to the existing support for color scheme

According to this MDN page

https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-reduced-motion#user_preferences

...there is ui.prefersReducedMotion that can be set to either 0 for full animation or to 1 to indicate a preference for reduced motion.

I filed the following bug, so this idea isn't lost

https://bugzilla.mozilla.org/show_bug.cgi?id=1811811

Thank you!

Honza