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/workflows/sync_files.yml

@@ -8,8 +8,8 @@ on:
     paths:
       - "build.gradle"
       - "README.md"
-      - "src/main/resources/messages_*.properties"
-      - "src/main/resources/static/3rdPartyLicenses.json"
+      - "stirling-pdf/src/main/resources/messages_*.properties"
+      - "stirling-pdf/src/main/resources/static/3rdPartyLicenses.json"
       - "scripts/ignore_translation.toml"
 
 permissions:
@@ -41,11 +41,11 @@ jobs:
 
       - name: Sync translation property files
         run: |
-          python .github/scripts/check_language_properties.py --reference-file "src/main/resources/messages_en_GB.properties" --branch main
+          python .github/scripts/check_language_properties.py --reference-file "stirling-pdf/src/main/resources/messages_en_GB.properties" --branch main
 
       - name: Commit translation files
         run: |
-          git add src/main/resources/messages_*.properties
+          git add stirling-pdf/src/main/resources/messages_*.properties
           git diff --staged --quiet || git commit -m ":memo: Sync translation files" || echo "No changes detected"
 
       - name: Install dependencies
@@ -101,4 +101,4 @@ jobs:
           sign-commits: true
           add-paths: |
             README.md
-            src/main/resources/messages_*.properties
+              stirling-pdf/src/main/resources/messages_*.properties