Move --help option defaults from its class to its decorator

[kdeldycke]

This is a proposal to fix #2832, which points to a subtle change in behavior of the help option introduced in 8.1.8.

The root of the problem is that by deduplicating code in 8.1.8 (see: #2563), I introduced a HelpOption class, to centralize the setup necessary to make a standard Option behave like an help option.

In #2832, a user is relying on a custom class to feed the @help_option decorator. This bypass all the custom setup made by HelpOption, and produce the discrepancy in behavior between 8.1.7 and 8.1.8.

This PR:

• Reverts back to setting up all parameters of the help option in the decorator instead of the HelpOption class
• Aligns @help_option implementation with the way Click is doing for the others (like @password_option, @version_option, ...)
• Removes the click.decorators.HelpOption class as a consequence
• Adds a complete unit test to check combination of custom class and option names leads to expected results

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.