💥 Replace exec-path-from-shell w/ 'bin/doom env'

IMPORTANT: This is a breaking update for Mac users, as your shell
environment will no longer be inherited correctly (with the removal of
exec-path-from-shell). The quick fix is: 'bin/doom env refresh'. Also,
the set-env! autodef now does nothing (and is deprecated), be sure to
remove calls to it in your config.

Smaller changes:
+ This update also adds --no-* switches to doom quickstart
+ Includes general improvements to the documentation of several bin/doom
  commands.
+ Moves doom/reload* commands to core/autoload/config.el
+ doom/reload-project has been removed (it didn't actually do anything)

The breaking change:
This update adds an "envvar file" to Doom Emacs. This file is generated
by `doom env refresh`, populated with variables scraped from your shell
environment (from both non-interactive and interactive sessions). This
file is then (inexpensively) loaded at startup, if it exists.

+ The file is manually generated with `doom env refresh`.
+ It can be regenerated automatically whenever `doom refresh` is run by
  running `doom env enable` (`doom env clear` will reverse this and
  delete the env file).
+ `doom quickstart` will ask if you want to auto-generate this envvar
  file. You won't need it if you're confident Emacs will always be
  started from the correct environment, however.
+ Your env file can be reloaded from a running Emacs session with `M-x
  doom/reload-env`. Note: this won't work if the Emacs session you're
  running it in doesn't have a correct SHELL set. i.e. don't use this to
  create your first env file!

The idea isn't mine -- it's borrowed from Spacemacs -- and was
introduced to me in #1053 by @yurimx. I was impressed with it. Prior to
this, I was unhappy with exec-path-from-shell (no hate to the dev, I
understand its necessity), and 'doom patch-macos' wasn't ideal for mac
users (needed to be reapplied every time you update Emacs). What's more,
many users (even Linux users) had to install exec-path-from-shell
anyway.

This solution suffers from none of their shortcomings. More reliable
than patch-macos, more performant and complete than
exec-path-from-shell, and easily handled by bin/doom.

modules/lang/ruby/config.el

@@ -15,7 +15,6 @@
         (enh-ruby-mode)
       (ruby-mode)))
   :config
-  (set-env! "RBENV_ROOT")
   (set-electric! '(ruby-mode enh-ruby-mode) :words '("else" "end" "elsif"))
   (set-repl-handler! '(ruby-mode enh-ruby-mode) #'inf-ruby)