devutils: add he exit command, fix sourcing dev.sh in zsh

[alexng353]

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• added he exit (this should probably be upstreamed)
• fixed sourcing dev.sh in zsh (used to use bash-only variables not available in zsh)

[alexng353]

I decided not to upstream these because there is a big diversion between the macos and linux dev scripts. Not sure how maintainers want to proceed with this.