Deprecation of previous exports from monolith

[semohr]

This PR introduces a deprecation system to clean up the public API and prepares for the v1.0.0 release.

Note: While not entirely necessary, I moved the MediaFile class into its own file. This cleans the __init__.py file to be responsible for import/export only. This simplified the deprecation process significantly and makes very clear what is exposed by mediafile going forward.

Changes:

• Added deprecation system with version-aware warnings for imports
• Simplified __init__.py to expose only core API (MediaFile, Image, exceptions) in its __all__
• Moved MediaFile class to dedicated file for better organization
• Adjusted tests to use the new structs to omit warnings

Migration:
Existing imports still work but raise a warnings:

import warnings
warnings.simplefilter("always")
from mediafile import StorageStyle
>>> DeprecationWarning: 'mediafile.StorageStyle' is deprecated and will be removed in version '1.0.0'. Use 'mediafile.storage.StorageStyle' instead.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.60000% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.94%. Comparing base (7b53be9) to head (ea3101a).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
mediafile/mediafile.py	89.83%	15 Missing and 9 partials ⚠️
mediafile/deprecation.py	50.00%	6 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #98      +/-   ##
==========================================
- Coverage   93.26%   91.94%   -1.33%     
==========================================
  Files          16       18       +2     
  Lines         817     1067     +250     
  Branches      118      156      +38     
==========================================
+ Hits          762      981     +219     
- Misses         35       56      +21     
- Partials       20       30      +10     

Files with missing lines	Coverage Δ
mediafile/deprecation.py	50.00% <50.00%> (ø)
mediafile/mediafile.py	89.83% <89.83%> (ø)