elisp lacks an execv implementation (or mature subprocess library), so
we exploit some splenderiffic hackery to get Emacs to execute arbitrary
shell commands after a 'doom ...' command completes. This allows us to
daisy chain doom commands in distinct sessions (wonderful for reloading
doom after a 'doom upgrade', which we do). This minimizes errors when a
'doom upgrade' pulls in breaking changes to Doom's CLI.
We also bring 'doom run' into elisp, since this new functionality
enables us to.