app-shells/atuin: add 18.4.0

Removed BDEPEND = dev-libs/protobuf, since upstream switched to protox.
See atuinsh/atuin#2122

Signed-off-by: Jonas Frei <[email protected]>
Closes: #39891
Signed-off-by: Florian Schmaus <[email protected]>

Author: freijon