Refactor: extract logic in Options.__init__ into helper methods

[victorletichevsky]

This pull request contributes to issue #5917, which encourages the decomposition of overly long methods to improve code clarity and maintainability.

Summary of changes

Refactored Options.__init__ by extracting its logic into several private helper methods:

• _initialize_caches
• _initialize_build_options
• _initialize_disallow_options
• _initialize_warning_and_reporting_options
• _initialize_misc_behavior
• _initialize_error_code_controls
• _initialize_target_definitions
• _initialize_caching_options
• _initialize_integration_flags
• _initialize_development_options
• _initialize_test_options
• _initialize_experimental_options

Notes

• All logic remains unchanged.
• Original comments have been preserved and relocated appropriately.

This is a pure refactor in line with issue #5917.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[JukkaL]

A long __init__ method that initializes attributes is arguably one case where splitting into multiple methods is not desirable, since our informal convention is to define all attributes in __init__. In the future we might even introduce a check to generate an error if an attribute is defined outside __init__ or the class body.

Defining attributes in the class body is not a good workaround either, since then we'd have declarations and initializations far away from each other, which is again not ideal. So unfortunately this is probably one of the few long methods where splitting it up doesn't seem worth it.