Drop module level mutability for GIL

[aldroid1]

• Drop shared module-level mutable state to avoid using GIL across calls. Simply means that any custom configuration must be assigned per instance and not globally.

• Introduce a builder class FuzzyDate that supports existing configuration and conversions. Direct functions remain the identical, calling the builder internally.

import fuzzydate as fd

result1 = (fd.FuzzyDate()
    # .add_patterns({...})
    # .add_tokens({...})
    # .set_first_weekday_sunday(True)
    .to_date('yesterday'))
    
result2 = fd.to_date('...')

assert result1 == result2