The CL add two files:
1) copy.bara.sky - source file for copybara that needs to migrate cppgc
from v8 repo to GitHub;
2) export_to_github.sh - top level bash script that runs copybara and
generates CMake files if needed.

Bug: v8:10724
Change-Id: I6a5f0ed1d007533914d578203a37fab2c3e9774a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2320651Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69123}