refactor!: replace all-the-icons with nerd-icons

BREAKING CHANGE: This commit replaces all-the-icons with nerd-fonts. Any
all-the-icons-* function calls or variable references in your private
config will break and should be replaced with their nerd-icons-*
equivalent. That said, Doom will continue to install all-the-icons for
a while, so feel free to load it if you don't want to fully commit to
the change yet.

This change is happening because nerd-icon has wider support for GUI and
TUI Emacs; has a larger, more consistent selection of symbols; plus unicode
coverage.

Fix: #7368
Close: #6675
Close: #7364

modules/app/everywhere/config.el

@@ -20,7 +20,7 @@
        (doom-modeline-spc)
        (when (emacs-everywhere-markdown-p)
          (concat
-          (all-the-icons-octicon "markdown" :face 'all-the-icons-green :v-adjust 0.02)
+          (nerd-icons-octicon "nf-oct-markdown" :face 'nerd-icons-green :v-adjust 0.02)
           (doom-modeline-spc)))
        (propertize (emacs-everywhere-app-class emacs-everywhere-current-app)
                    'face 'doom-modeline-project-dir)