Rename webgpu_cpp.cpp to api_cpp.cpp and make it flexible.
BUG=dawn:1201
Change-Id: Ib964be9399e9733c4fe9de5d1cd0361182bb0836
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/72460
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Junwei Fu <junwei.fu@intel.com>