Add Gemini context files

These are copied and adapted from Chrome.

Because src/dawn and src/tint are really two separate codebases with
different coding standards, tooling, ways to update build files and to
run tests, I've split the common and knowledge_base files into two, one
for each codebase. This should allow for us to add project-specific
stuff at the right place.

Bug: 492219316
Change-Id: I9fef1cc350b8172d41cc104cfb8069e3f59b7c6a
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/297555
Reviewed-by: Gregg Tavares <gman@chromium.org>
Reviewed-by: James Price <jrprice@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>