Drop FTL stub generator

[andrew000]

Drop FTL stub generator due to having better FTL-Extract ftl stub, that uses python ast lib to generate stubs

[Copilot]

Pull Request Overview

This PR removes the built-in FTL stub generator implementation in favor of the external ftl-extract package, which provides better functionality using Python's ast library.

Key Changes

• Completely removes the FluentVisitor implementation that parsed Fluent translation files
• Replaces from_files_to_file_ex functionality with a deprecation message directing users to ftl-extract
• Updates deprecation messages in CLI commands to include usage examples

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
aiogram_i18n/utils/fluent_stub/visitor.py	Deleted entire FluentVisitor class that handled Fluent file parsing
aiogram_i18n/utils/fluent_stub/init.py	Replaced stub generation logic with deprecation notice pointing to ftl-extract
aiogram_i18n/utils/cli/multiple_extract.py	Enhanced deprecation message with ftl-extract usage example
aiogram_i18n/utils/cli/extract.py	Enhanced deprecation message with ftl-extract usage example

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