Add transient-pointer targetRayMode (#33740)

See:
https://www.w3.org/TR/webxr/#dom-xrtargetraymode-transient-pointer

files/en-us/web/api/xrinputsource/targetraymode/index.md

@@ -32,6 +32,8 @@ the user. The possible values are:
   - : The direction of the target ray is indicated using a tap on a touch screen, mouse, or other tactile input device.
 - `tracked-pointer`
   - : Targeting is being performed using a handheld device or hand-tracking system which the user points in the direction of the target. The target ray extends from the hand (or the object in the hand) in the targeted direction. The direction is determined using platform-specific rules, though if no such rules exist, the direction is chosen by assuming the user is pointing their index finger straight out from their hand.
+- `transient-pointer`
+  - : The input source was generated as part of an operating system interaction intent rather than a specific piece of hardware. Some examples include user intents based on information too sensitive to expose directly such as gaze, synthesized inputs from web drivers, or inputs generated by assistive technologies.
 
 ## Usage notes