feat(emacs.d): Introduce custom mc/mark-dwim cursor marker

Adds a "do what I mean" multiple-cursor selection with the logic that
I find most useful:

* If there is no active region, mark the next line (or lines, based on
  prefix argument)
* If there is an active region that spans multiple lines, call
  `mc/edit-lines`
* If there is an active region on a single line, trigger a custom
  selection hydra with functionality equivalent to
  `mc/mark-more-like-this-extended` but a slightly improved user
  experience

Hopefully this will make it easier to get into the habit of actually
using multiple-cursors without calling the mc commands via M-x

tools/emacs/config/bindings.el

@@ -24,6 +24,7 @@
 ;; Miscellaneous editing commands
 (global-set-key (kbd "C-c w") 'whitespace-cleanup)
 (global-set-key (kbd "C-c a") 'align-regexp)
+(global-set-key (kbd "C-c m") 'mc/mark-dwim)
 
 ;; Browse URLs (very useful for Gitlab's SSH output!)
 (global-set-key (kbd "C-c b p") 'browse-url-at-point)