Introduces a script to generate Blit11's
BlitShaderType enum and shader mapping functions.

Bug: angleproject:2870
Change-Id: I5b704f55bdde1706614a519ecb0ee3756f7799e0
Reviewed-on: https://chromium-review.googlesource.com/c/1273725Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>