Automatically switch from RSpec compilation mode to interact with a debugger

This commit adds the 'inf-ruby-auto-enter method to
'compilation-filter-hook, automatically entering inf-ruby-mode when it
hits a breakpoint (see the
[inf-ruby-manual](https://github.com/nonsequitur/inf-ruby/#manual) for
more details).

The hook is added locally to the rspec-compilation-mode-hook to avoid
global slowdown, as the compilation-filter-hook is called whenever a
new line is entered.

layers/+lang/ruby/packages.el

@@ -127,6 +127,10 @@
     ;; setup the hook via an autoload
     :config
     (progn
+      (defun spacemacs//inf-ruby-auto-enter ()
+        "Automatically enters inf-ruby-mode in ruby modes' debugger breakpoints."
+        (add-hook 'compilation-filter-hook 'inf-ruby-auto-enter nil t))
+      (add-hook 'rspec-compilation-mode-hook 'spacemacs//inf-ruby-auto-enter)
       (spacemacs|hide-lighter rspec-mode)
       (dolist (mode '(ruby-mode enh-ruby-mode))
         (spacemacs/set-leader-keys-for-major-mode mode