Add `--save-bundle <dir>` to help and to README

[unional]

Found this and it is still working:
#95

but it is not mentioned in the size-limit --help nor in the README. 🌷

[ai]

Do you want to add your name in the project history and send PR?

[unional]

Either is fine. :)

I noticed that it is actually under webpack. But it also works for esbuild. I have a project that does not have webpack so the cli does not show that help, but it is still working as expected.

Maybe move it up to the core options?