Add .gitattributes for package distribution

Excludes development and documentation files from Composer package:
- Development configs (.editorconfig, .gitignore, .gitattributes)
- GitHub workflows
- Documentation folder
- Test files and configs
- Vendor directory

This is, of course, excluding things that are not yet part of the repository but will likely be part of it soon.

Author: borkweb

.gitattributes

@@ -0,0 +1,9 @@
+.editorconfig          export-ignore
+.gitattributes         export-ignore
+/.github/              export-ignore
+.gitignore             export-ignore
+/docs/                 export-ignore
+/tests/                export-ignore
+/vendor/               export-ignore
+/phpstan.neon.dist     export-ignore
+/phpunit.xml.dist      export-ignore