automation: Pressure types supported in WebDriver are the same as what PressureObserver.knownSources returns

[rakuco]

In other words, if an implementation does not have a source type in knownSources, one is also unable to add a virtual pressure source of said type even though the samples are entirely user-controlled.

This issue tracks whether this is acceptable behavior or not. If it is not, we also need to think of whether knownSources needs to return a different value depending on the virtual pressure sources that have been created.