Add tryparse support for URI references

[quinnj]

Summary

• implement Base.tryparse(URI, str)
• return nothing only for URIs.ParseError
• document absolute and relative URI-reference behavior
• add success, relative-reference, and malformed-input tests

Closes #60.

Validation

• Julia 1.6.7: full test suite passed (253 URI tests, 12 URL tests)
• Julia 1.12.6: full test suite passed (253 URI tests, 12 URL tests)
• Documenter build passed

Co-authored by Codex

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.16%. Comparing base (0f4ecc3) to head (af85f33).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
+ Coverage   84.63%   85.16%   +0.52%     
==========================================
  Files           5        5              
  Lines         332      337       +5     
==========================================
+ Hits          281      287       +6     
+ Misses         51       50       -1     

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