This adds a new script `update-mirror.sh` which generates the proper
blob format for R2 (or any blob storage) to mirror all of our
dependencies.
It doesn't automate updating build.zig.zon but on an ongoing basis this
should be easy to do manually, and we can strive to automate it in the
future.
I omitted iTerm2 color themes because we auto-update that via CI and
updating all of the machinery to send it to our mirror and so on is a
pain. Additionally, this doesn't mirror transitive dependencies because
Zig doesn't have a way to fetch those from a mirror instead (unless you
pre-generate a full cache like packagers but that's not practical for
day to day development).
It's hugely beneficial just to get most of our dependencies mirrored.
