[actions] Specify how pointers are assigned a pointerId

[jgraham]

Pointer events get a pointerID property. The ID must be unique per pointer, and the Pointer Events spec has some requirements, but WebDriver BiDi lacks specific text on how to assign ids to virtual pointers. Basically the rule should be that pointer ids are handed out in order starting from 0, except that 0 and 1 are reserved for mouse-type pointers.