add regression for debug_sources Buffer rethrow (#3290)

[dhirajlaulkar]

• Add packages/pug/test/rethrow-buffers.test.js with fixtures to ensure
  included sources are stored as strings when compileDebug is enabled,
  preventing pug_rethrow from calling .split on a Buffer
  (reproduces and guards against Generated debug_sources are node Buffers and fail pug_rethrow #3290).
• Update packages/pug/test/error.reporting.test.js to accept both legacy
  and Node 16+ error message formats:
  • "Cannot read property 'length' of X"
  • "Cannot read properties of X (reading 'length')"

All test suites pass locally after these changes.

Refs: #3290

[rollingversions]

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