Add optional only_show_changed flag to show changed files only

This is a non-breaking new feature that addresses issue #129.

New input parameter:
- only_show_changed (default: false)
  When true, shows only files that were changed by Prettier instead of
  all processed files. Useful for large projects where output is
  overwhelming.

Backward compatibility:
- When flag is NOT set or is false: behavior stays exactly as before
  (shows "Files:" header and all prettier output)
- When flag is set to true: shows only "Changed files:" with git status
  output for modified files

In projects with ~2,000 files, it's hard to see which were changed.
This optional flag makes it easy to identify modified files without
affecting existing users who rely on the current output format.

Closes #129
Refs prettier/prettier#2611

action.yml

@@ -74,6 +74,10 @@ inputs:
     description: Allow other plugins to be installed. By default, we are checking if the plugins are actually prettier plugins.
     required: false
     default: false
+  only_show_changed:
+    description: Only show files that were changed by Prettier instead of all processed files. Useful for large projects where output is overwhelming.
+    required: false
+    default: false
 
 runs:
   using: "composite"
@@ -102,6 +106,7 @@ runs:
         INPUT_CLEAN_NODE_FOLDER: ${{ inputs.clean_node_folder }}
         INPUT_GIT_IDENTITY: ${{ inputs.git_identity }}
         INPUT_ALLOW_OTHER_PLUGINS: ${{ inputs.allow_other_plugins }}
+        INPUT_ONLY_SHOW_CHANGED: ${{ inputs.only_show_changed }}
 
 branding:
   icon: "award"