Multi module refactor (#3640)

# Description of Changes

Migrated Stirling PDF to a multi-module structure:

* Introduced new `:stirling-pdf` module
* Moved all the core logic and features of Stirling PDF into
`:stirling-pdf`
* Updated paths of jobs and scripts

---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#6-testing)
for more details.

.github/scripts/check_language_properties.py

@@ -317,7 +317,7 @@ def check_for_differences(reference_file, file_list, branch, actor):
         report.append("## ❌ Overall Check Status: **_Failed_**")
         report.append("")
         report.append(
-            f"@{actor} please check your translation if it conforms to the standard. Follow the format of [messages_en_GB.properties](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/src/main/resources/messages_en_GB.properties)"
+            f"@{actor} please check your translation if it conforms to the standard. Follow the format of [messages_en_GB.properties](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/stirling-pdf/src/main/resources/messages_en_GB.properties)"
         )
     else:
         report.append("## ✅ Overall Check Status: **_Success_**")