release.zsh: Add `emulate zsh` line.

In `scripts/release.zsh`, add an `emulate zsh` command at the start of
the script.
release.zsh: Fix an error message.

In `scripts/release.zsh`, I had forgotten to change the error message…

    ‘$pbl’ does not exist or is not a regular file.

…to reflect that the “does not exist” part had been split into a
separate check.

This commit fixes the issue by deleting the “does not exist” part.
release.zsh: Allow for releasing a new add-on.

In `scripts/release.zsh`, allow for the release of an add-on that is not
already on the targeted add-ons server.