Support emacs 27+ early init

[lastquestion]

Currently, when we notice we are in init, we install during emacs-startup-hook, which is extremely late.

The reason why is because someone could run advised-code in init.el or as a -f command arg, and because explain-pause-mode itself was also called during that same phase, we cannot guarentee we have a clean state to install all hooks and setup the command stack.

• Document that early init would allow you to see what happens during init.el
• Update the init detection code to handle early init and install anyway.
• Perhaps set a flag during all init phase so that if report-bug fires, we know it was during init.