Improve help listing for regex aliases

This commit improves the appearance of regex aliases in the help index
by storing the result of calling #inspect on the regex as the listing.

For example, consider the `whereami` alias `/whereami[!?]+/`.
Previously this would appear in the help index as
`(?-mix:whereami[!?]+)`. This commit fixes this so it appears as
`/whereami[!?]+/`.

lib/pry/command_set.rb

@@ -193,7 +193,7 @@ def alias_command(match, action, options = {})
 
       options = original_options.merge!(
         desc: "Alias for `#{action}`",
-        listing: match
+        listing: match.is_a?(String) ? match : match.inspect
       ).merge!(options)
 
       # ensure default description is used if desc is nil

spec/command_set_spec.rb

@@ -185,11 +185,16 @@
       expect(new_command.description).to eq('Alias for `test`')
     end
 
-    it "sets aliased command's listing" do
+    it "sets aliased command's listing for string alias" do
       new_command = subject.alias_command('new-test', 'test')
       expect(new_command.options).to include(listing: 'new-test')
     end
 
+    it "sets aliased command's listing for regex alias" do
+      new_command = subject.alias_command(/test[!?]+/, 'test')
+      expect(new_command.options[:listing].to_s).to eq('/test[!?]+/')
+    end
+
     it "sets group for the aliased command automatically" do
       new_command = subject.alias_command('new-test', 'test')
       expect(new_command.group).to eq('Aliases')