Add to_s representation to LintRoller::Plugin`

While developing rubocop/rubocop#13792,
I noticed that `LintRoller::Plugin` lacks a string representation.
For example, in RuboCop, there is a feature to display a list of plugins using the `rubocop -V` command:

```console
$ bundle exec rubocop -V
1.71.2 (using Parser 3.3.7.0, rubocop-ast 1.38.0, analyzing as Ruby 2.7, running on ruby 3.4.1) [x86_64-darwin23]
  - rubocop-performance 1.23.1
  - rubocop-rake 0.6.0
  - rubocop-rspec 3.4.0
```

In cases like this, having a string representation would simplify things.

I'm not sure what the best string representation would be, but I have chosen to include the name and version as
they provide the minimal and most easily identifiable details.

Author: koic

lib/lint_roller/plugin.rb

@@ -18,5 +18,15 @@ def supported?(context)
     def rules(context)
       raise Error.new("Please implement `rules(context)` and return an instance of LintRoller::Rules")
     end
+
+    def to_s
+      if about.name && about.version
+        "#{about.name} #{about.version}"
+      elsif about.name
+        about.name
+      else
+        inspect
+      end
+    end
   end
 end

test/lib/plugin_test.rb

@@ -58,6 +58,8 @@ def test_sample_roller
       assert sample_roller.supported?(Context.new(runner: :standard))
       refute sample_roller.supported?(Context.new(runner: :rufo))
 
+      assert_equal sample_roller.to_s, 'sample-roller 1.2.3'
+
       assert_equal Rules.new(
         type: :path,
         config_format: :rubocop,