Add readline-ext that has been removed from Ruby 3.3.0dev

Address these Rails CI failures:
https://buildkite.com/rails/rails/builds/96301#01880117-acb1-4d77-935a-4a1de13c8609/1093-1101

Refer to
https://bugs.ruby-lang.org/issues/19616
ruby/ruby#7781

```ruby
$ ruby -v
ruby 3.3.0dev (2023-05-09T19:40:26Z master e1c84e8ee6) [x86_64-linux]
$ cd railties
$ bin/test test/application/console_test.rb
... snip ...

.F

Failure:
FullStackConsoleTest#test_environment_option_and_irb_option [/home/yahonda/src/github.com/rails/rails/railties/test/application/console_test.rb:133]:
"> " expected, but got:

Loading test environment (Rails 7.1.0.alpha)
Switch to inspect mode.
▽.
Expected # encoding: ASCII-8BIT
"Loading test environment (Rails 7.1.0.alpha)\r\nSwitch to inspect mode.\r\n\e[1G\xE2\x96\xBD\e[6n" to include "> ".

bin/test test/application/console_test.rb:167

^[[30;2RF

Failure:
FullStackConsoleTest#test_sandbox [/home/yahonda/src/github.com/rails/rails/railties/test/application/console_test.rb:133]:
"> " expected, but got:

Loading development environment in sandbox (Rails 7.1.0.alpha)
Any modifications you make will be rolled back on exit
▽.
Expected # encoding: ASCII-8BIT
"Loading development environment in sandbox (Rails 7.1.0.alpha)\r\nAny modifications you make will be rolled back on exit\r\n\e[1G\xE2\x96\xBD\e[6n" to include "> ".

bin/test test/application/console_test.rb:139

^[[30;2R......

Finished in 66.153595s, 0.1360 runs/s, 0.2872 assertions/s.
9 runs, 19 assertions, 2 failures, 0 errors, 0 skips
$
```

Gemfile

@@ -75,6 +75,7 @@ gem "bootsnap", ">= 1.4.4", require: false
 gem "webrick", require: false
 gem "jbuilder", require: false
 gem "web-console", require: false
+gem "readline-ext", require: false
 
 # Action Pack and railties
 rack_version = ENV.fetch("RACK", "~> 2.0") # Change to ~> 3 after #46594 is merged.

Gemfile.lock

@@ -398,6 +398,7 @@ GEM
     rbtree (0.4.6)
     rdoc (6.5.0)
       psych (>= 4.0.0)
+    readline-ext (0.1.5)
     redcarpet (3.2.3)
     redis (5.0.5)
       redis-client (>= 0.9.0)
@@ -607,6 +608,7 @@ DEPENDENCIES
   rails!
   rake (>= 13)
   rdoc (~> 6.5)
+  readline-ext
   redcarpet (~> 3.2.3)
   redis (>= 4.0.1)
   redis-namespace