more robust detection of Bun

Because of https://gitlab.com/soapbox-pub/xjsr/-/blob/main/bin/xjsr.js?ref_type=heads#L1 bunx will actually run node unless its run with --bun , bunx --bun @repo/repo

This PR fixes that by detecting Bun even with the hashbang

Summary by CodeRabbit

• Bug Fixes

  • Improved Bun runtime detection so the app reliably recognizes Bun environments, including processes launched via bunx, preventing misidentification and ensuring consistent runtime behavior.

• Documentation

  • Updated README with a note clarifying that the project correctly detects and uses Bun when executed with bunx.

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