Use pronto.yml to configure pronto-stylelint

This is necessary since pronto-stylelint 0.11.0, which we started using
in commit 2768263fa. We were getting an error when runnin Pronto in pull
requests that changed CSS code:

```
bundler: failed to load command: pronto (~/.rbenv/versions/3.2.7/bin/pronto)
~/.rbenv/versions/3.2.7/lib/ruby/3.2.0/open3.rb:222:in `spawn': No such
file or directory - stylelint (Errno::ENOENT)
```

.pronto.yml

@@ -2,3 +2,5 @@ rubocop:
   suggestions: true
   severities:
     refactor: info
+stylelint:
+  stylelint_executable: "npx stylelint"

.pronto_stylelint.yml

@@ -1 +0,0 @@
-stylelint_executable: "npx stylelint"