Automation: Remove "virtual sensor type" from virtual sensor metadata

[rakuco]

This practically untangles the concepts in the Automation section from the rest of the specification, which helps the Device Orientation API specification adopt it as discussed in w3c/deviceorientation#122.

Now a sensor type itself can optionally define a "virtual sensor type", a string. It is the same string used to refer to a given virtual sensor type in the WebDriver extension commands.

In practice, this means that:

• The per-top-level traversable virtual sensor mapping has become a mapping of virtual sensor types to virtual sensor.
• The per-type virtual sensor metadata mapping has become a mapping of virtual sensor types to virtual sensor metadata.
• Sensor types that define a virtual sensor type attempt to retrieve a virtual sensor from the virtual sensor mapping using this string as key.

There are no user-visible changes or changes to the WebDriver extension commands, it is only the concepts and data structures used internally in the specification that have changed a little.

Derived specifications will need to be adjusted accordingly once this pull request is merged.

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[rakuco]

(FWIW I think this change makes sense regardless of w3c/deviceorientation#124)