feat(git)!: enable async git prompt (now for real)

BREAKING CHANGE: the new async prompt feature will render prompt information
asyncronously and degrade nicely when the calls take too long to finish, as
the prompt will already be first drawn and interactive. This is
enabled by default for the git prompt and themes that use it (`git_prompt_info`).
If you find that it's not working for you, please open an issue if one is
not already opened, and see https://github.com/ohmyzsh/ohmyzsh#disable-async-git-prompt
for how to turn it off.