Improve URI component updates

[quinnj]

Summary

• let nothing remove any URI component while preserving the meaning of empty strings
• support authority components for arbitrary URI schemes
• preserve the // authority delimiter when a URI has no scheme
• document component updates, the cached .uri field, and query-pair updates
• clarify joinpath versus resolvereference and path strings versus URI strings
• improve component-specific constructor error messages
• add regression tests for each removable component and generic schemes

Closes #21.
Closes #22.
Closes #23.
Closes #24.
Closes #26.
Closes #29.
Closes #30.
Closes #34.
Closes #37.
Closes #65.

Validation

• Julia 1.6.7: full test suite passed (260 URI tests, 12 URL tests)
• Julia 1.12.6: full test suite passed (260 URI tests, 12 URL tests)
• Documenter build and doctests passed
• changed URI forms round-trip through string and URI

Co-authored by Codex

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.00%. Comparing base (0f4ecc3) to head (d99e572).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
+ Coverage   84.63%   85.00%   +0.36%     
==========================================
  Files           5        5              
  Lines         332      340       +8     
==========================================
+ Hits          281      289       +8     
  Misses         51       51              

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