Introduce enumeration(s) for well known technology platforms

[danbri]

In #1150 @betehess proposes ActionPlatform as a class, to accompany the existing https://schema.org/actionPlatform

However we also have a similar construct on VideoGame and VideoGameSeries, i.e. https://schema.org/gamePlatform - this property is currently super open-ended (Text/URL/Thing).

There are also some unofficial terms in widespread use dating from the days before we had the "Pending" mechanism for proposals: http://schema.org/DesktopWebPlatform http://schema.org/MobileWebPlatform http://schema.org/AndroidPlatform http://schema.org/IOSPlatform. To these I would suggest we add a general /WebPlatform, since much content now tries to target both mobile and desktop; not to mention the other environments the Web is available from (e.g. WebXR browsers).

I suggest we start with an enumeration that can be used by both actionPlatform and gamePlatform, and be open to adding new codes to it as they're suggested here.

[danbri]

On closer inspection, the granularity expected for something like game platforms is quite different (e.g. see this wikidata list). However I believe we're better off with an enumerated list that can be extended in an ad hoc manner, rather than using classes here. It's also probably a good idea to not tie this solely to actions, so I'm using DigitalPlatformEnumeration but with disclaimers to emphasise that the list is a starting place rather than a complete enumeration.

[github-actions]

This issue is being nudged due to inactivity.