feat: Add Storage.php for handling file reading/writing

[sevaske]

• Created "Storage" for handing working with files.
• Removed unnecessary echo statement in saveXMLFile.
• Fixed issue when saving a file with a specific path without using "output".
• Fixed file path handling issue on Windows OS.
• Fixed "$json_data" ([0] issue) in README.md.

For tests:
composer test tests/StorageTest.php

[sevaske]

I had an issue when calling saveXMLFile(storage_path('signed.xml')) because "output" was being added to the file path inside saveXMLFile. I kept "output" in the saveXMLFile function to support existing.
public function saveXMLFile(string $filename = 'signed_invoice.xml', ?string $outputDir = 'output'): self
This ensures that if you use saveXMLFile({full path}, null), it will work correctly.