Add support for merging header-timers with existing Server-Timing headers

[tbeseda]

[note from tbeseda (the rest is Copilot): I'm testing out Copilot Workspace trial]

Related to #1

Adds support for merging header-timers values with existing Server-Timing headers without overwriting, and updates documentation and tests accordingly.

• New Feature Implementation:
  • Introduces a mergeWithExisting method in index.js that allows merging header-timers values with existing Server-Timing headers, supporting both string and object formats for existing headers.
  • Modifies the toObject method to facilitate merging by handling existing headers passed as either a string or an object, including cases where the Server-Timing header is already present.
• Documentation Updates:
  • Enhances the README.md file with a new section and examples on how to use the mergeWithExisting method for merging header-timers values with existing Server-Timing headers.
• Testing Enhancements:
  • Adds tests in index-test.js to verify the correct functionality of the mergeWithExisting method, including tests for merging with existing headers provided as both strings and objects.

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