Add XRSessionMode type to BCD (#4964)
Add XRSessionMode type to BCD (#4964)

* Add XRSessionMode type to BCD
Adds a BCD file for the `XRSessionMode` enum. The
values each have a feature entry, with the URLs
pointing to the anchor for each value in the
`XRSessionMode` page (since we don't do subpages for
values of enums).

Source: https://immersive-web.github.io/webxr/#xrsessionmode-enum

* Updates per feedback from jpmedley

mdn/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data