Don't treat the default key as special in createUtilityPlugin(), add mapTheme argument and convertDefaultKey helper function for plugins that need it

[benface]

As discussed on Discord. This fixes the "issue" that a default key in the transitionDuration theme object generates a duration class instead of duration-default which makes more sense.

[benface]

@adamwathan Do you consider this a breaking change? I'm using a fork of Tailwind in some projects just for this, and wondering if I should just use another key for the default transition duration.

[adamwathan]

Decided to solve this in a different/simpler way, thanks for this though, always appreciate your input on this stuff.

#2580