feat(updater): add `background-alpha` update mode (preview) (#11928)

NOTE: this feature is in alpha / preview mode, it is not guaranteed to work 100% of the
time in all cases. If you experience any issues, open an issue or search for an open
one describing your same situation.

To use this, use the zstyle update mode settings [1] with the value `background-alpha`:

  zstyle ':omz:update' mode background-alpha

[1] https://github.com/ohmyzsh/ohmyzsh#getting-updates