editorial: Modernize and replace prose with algorithms in BatteryManager

[rakuco]

Similarly to #45, make the BatteryManager section more conformant to modern spec writing practices, while avoiding making user-visible changes as much as possible:

• Define a task source for this spec, and switch from "queue a task" to the more specific and recent "queue a global task" algorithm.
• Replace a lot of prose with internal slots and algorithms.
• Make the interface attributes simply return the values of the internal slots rather than defining logic in their getters.
• Define an actual algorithm for updating internal slots and dispatching events. The existing prose just said "the UA must change the attribute and fire an event" without really defining how to reach a BatteryManager object in the first place.

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[rakuco]

@reillyeon @marcoscaceres This one is bigger than #45, so some extra pairs of eyes are appreciated.

[marcoscaceres]

Will try to review soon!

[anssiko]

I authorize @reillyeon to merge this PR once we get @marcoscaceres's eyeballs on it.

[rakuco]

thanks for reviewing, @reillyeon and @marcoscaceres

I'd appreciate it if someone could merge this PR

[anssiko]

Thanks @rakuco! You should add yourself to the editors if you plan to keep an eye on this spec. Otherwise, we’ll give you huge credits in the ack section for these modernizations.

[rakuco]

So far I've just been trying to catch up with the backlog of things I'd promised to do back in April during the DAS WG virtual meeting :-) Once I finish making the spec look better (and hopefully address #15) I can consider officially becoming an editor.