Add rubocop binstub for spring support

I observed a significant speed-up on the second run. The first run seems
to be a lot slower though. Additional output suggests that it's now
loading the Rails environment which it doesn't do without spring.

```
time bundle exec rubocop Gemfile
0m2.496s

time rubocop Gemfile # not always right version
0m1.999s

time ./bin/rubocop Gemfile
0m7.543s

time ./bin/rubocop Gemfile
0m0.670s
```

Gemfile

@@ -188,6 +188,7 @@ group :development do
   gem 'rubocop-rails'
   gem 'spring'
   gem 'spring-commands-rspec'
+  gem 'spring-commands-rubocop'
   gem 'web-console'
 
   gem 'rack-mini-profiler', '< 3.0.0'

Gemfile.lock

@@ -704,6 +704,8 @@ GEM
     spring (4.1.3)
     spring-commands-rspec (1.0.4)
       spring (>= 0.9.1)
+    spring-commands-rubocop (0.4.0)
+      spring (>= 1.0)
     sprockets (3.7.2)
       concurrent-ruby (~> 1.0)
       rack (> 1, < 3)
@@ -925,6 +927,7 @@ DEPENDENCIES
   spreadsheet_architect
   spring
   spring-commands-rspec
+  spring-commands-rubocop
   state_machines-activerecord
   stimulus_reflex (= 3.5.0.rc3)
   stimulus_reflex_testing

bin/rubocop

@@ -0,0 +1,4 @@
+#!/usr/bin/env ruby
+load File.expand_path("spring", __dir__)
+require 'bundler/setup'
+load Gem.bin_path('rubocop', 'rubocop')

bin/spring

@@ -1,6 +1,6 @@
 #!/usr/bin/env ruby
 
-# This file loads Spring without using loading other gems in the Gemfile, in order to be fast.
+# This file loads Spring without loading other gems in the Gemfile in order to be fast.
 # It gets overwritten when you run the `spring binstub` command.
 
 if !defined?(Spring) && [nil, "development", "test"].include?(ENV["RAILS_ENV"])