docs: tagfiles extension is npm package

[alandefreitas]

This PR moves the c++ tagfiles extension out of the Boost.URL so it can also be included by the playbook project in website-v2-docs and all components that might need it. The npm package also simplifies the dependencies and allows the extension to have its own documentation and unit tests.

[cppalliance-bot]

Antora version: an automated preview of the documentation is available at https://852.urlantora.prtest2.cppalliance.org/site/index.html

[cppalliance-bot]

An automated preview of the documentation is available at https://852.url.prtest2.cppalliance.org/libs/url/doc/html/index.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.21%. Comparing base (ab3cce2) to head (671cdd6).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #852   +/-   ##
========================================
  Coverage    99.21%   99.21%           
========================================
  Files          157      157           
  Lines         8422     8422           
========================================
  Hits          8356     8356           
  Misses          66       66           

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[cppalliance-bot]

GCOVR code coverage report https://852.url.prtest2.cppalliance.org/gcovr/index.html
LCOV code coverage report https://852.url.prtest2.cppalliance.org/genhtml/index.html
Coverage Diff https://852.url.prtest2.cppalliance.org/gcovr/coverage_diff.txt