Expose click API from cloup namespace

[janluke]

Closes #204

Summary

• Re-export the complete public Click 8.5 top-level namespace from Cloup while retaining the intentional Cloup overrides.
• Mirror Click's lazy deprecated aliases for as long as Click continues to provide them.
• Update examples and documentation to describe the unified namespace.
• Simplify the installation and dependency guidance.

Breaking changes

• cloup._version is no longer included in cloup.__all__. Consequently, from cloup import * won't import _version. This symbol is private and was added purely by mistake.

• cloup.warnings is no longer included in cloup.__all__. Consequently, from cloup import * will no longer import warnings and shadow the standard-library warnings module.

  Avoid from cloup import warnings, use:

  import cloup
  
  # example
  cloup.warnings.formatter_settings_conflict = False

Thanks @kdeldycke for the heads up!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.87%. Comparing base (27f5c74) to head (2fa0ab8).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #205      +/-   ##
==========================================
+ Coverage   96.86%   96.87%   +0.01%     
==========================================
  Files          22       22              
  Lines        1626     1633       +7     
==========================================
+ Hits         1575     1582       +7     
  Misses         51       51              

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