Add gem for spring to listen to filesystem changes efficiently

I found that each spring process was using around 3% CPU in the
background just scanning for file changes. By default, spring polls the
file system every 0.2 seconds.

With the added gem, I can't see any CPU use of spring in the background.

Gemfile

@@ -203,6 +203,7 @@ group :development do
   gem 'spring'
   gem 'spring-commands-rspec'
   gem 'spring-commands-rubocop'
+  gem 'spring-watcher-listen'
   gem 'web-console'
 
   gem 'rack-mini-profiler', '< 3.0.0'

Gemfile.lock

@@ -857,6 +857,9 @@ GEM
       spring (>= 0.9.1)
     spring-commands-rubocop (0.4.0)
       spring (>= 1.0)
+    spring-watcher-listen (2.1.0)
+      listen (>= 2.7, < 4.0)
+      spring (>= 4)
     sprockets (3.7.5)
       base64
       concurrent-ruby (~> 1.0)
@@ -1101,6 +1104,7 @@ DEPENDENCIES
   spring
   spring-commands-rspec
   spring-commands-rubocop
+  spring-watcher-listen
   sprockets (~> 3.7)
   state_machines-activerecord
   stimulus_reflex