ci: add .deepsource.toml

[deepsource-autofix[bot]]

This pull request adds a .deepsource.toml.

Merging it will successfully integrate DeepSource with this repository. On every subsequent pull request, it will run analysis and report any issues that need to be fixed. Good work!

[deepsource-io[bot]]

Here's the code health analysis summary for commits 90158b4..fc974b4. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
C#	✅ Success		View Check ↗

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