Add skeleton traverser for outputting WGSL Bug: angleproject:8662 Change-Id: I0d917c65692eb0f137c7e751f65f16312a4625f2 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5581744 Reviewed-by: Liza Burakova <liza@chromium.org> Commit-Queue: Matthew Denton <mpdenton@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>