Cleanup xrefObj on Dispose

[anisite]

Hello, here is a pull request to reduce memory usage after cleanup. In my tests with large files, memory usage at the end is 377 MB instead of 703 MB.

Fixes #143

[what-the-diff]

PR Summary

• Modification of PdfReader.cs file
  The PdfReader.cs file was updated, suggesting the team is refining the way we handle PDF files in our system.

• Addition of Dispose() method to PdfReader class
  A new functionality was introduced to properly discard any unused or redundant resources occupied by the PdfReader. This will help optimize our system's performance and efficiency.

• Enrichment of Dispose() method
  The Dispose() method now includes a step to clear out the _xrefObj list, ensuring our system avoids storing unnecessary or irrelevant data, again contributing to further optimization.

• Whitespace Adjustment
  There was a minor formatting adjustment, with a new line added at the end of the file. Although a subtle change, this could enhance the readability and maintenance of the code.