feat: add&prioritize zstd encoding and update tests/docs

[shoyuf]

• add explicit support for zstd in Accept-Encoding parsing
• set preferredEncodings order to [zstd, br, gzip, deflate]
• keep wildcardAcceptEncoding for Accept-Encoding: * limited to gzip and deflate
• update error message and directive regex to include zstd
• adjust tests to reflect new zstd preference and wildcard behavior
• clarify default behavior when no Accept-Encoding header is sent
• document zstd support and encoding priorities in the README

Checklist

• I have ensured my pull request is not behind the main or master branch of the original repository.
• I have rebased all commits where necessary so that reviewing this pull request can be done without having to merge it first.
• I have written a commit message that passes commitlint linting.
• I have ensured that my code changes pass linting tests.
• I have ensured that my code changes pass unit tests.
• I have described my pull request and the reasons for code changes along with context if necessary.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (0deefb8) to head (b9c814a).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #274   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          217       219    +2     
  Branches        36        36           
=========================================
+ Hits           217       219    +2     

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