Add Node.docstring and import alias helpers

[ggueret]

Summary

• Adds Node.docstring() (mixin method) returning the module/function/class docstring text or None.
• Adds iter_import_aliases() and the ImportAlias NamedTuple in rude.utils, absorbing the three import node types (plain, from, __future__).
• Re-exports extract_string_content, iter_import_aliases, ImportAlias at the top-level rude namespace.

Test plan

• Node.docstring() extracts from module, function, class; returns None when absent or non-eligible; handles triple-single-quote
• iter_import_aliases() yields correct (full_name, alias, is_from) for plain, aliased, dotted, from-import, multi-name, and __future__ imports
• Top-level from rude import extract_string_content, iter_import_aliases, ImportAlias works

[codecov]

Codecov Report

❌ Patch coverage is 74.64789% with 18 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
python/rude/utils.py	72.00%	14 Missing ⚠️
python/rude/core/node.py	80.00%	4 Missing ⚠️

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