Modernization and cleanup (#86)

This PR completely restructures the MediaFile library from a single
monolithic file into a modular, maintainable package architecture. The
core functionality remains unchanged while improving code organization,
readability, and future maintainability.

## **Architectural Restructuring**
- **Split monolithic `mediafile.py`** (2342 lines) into logical modules:
  - `mediafile/__init__.py` - Main MediaFile class and public API
  - `mediafile/constants.py` - Constants and enums (TYPES, ImageType)
  - `mediafile/exceptions.py` - Custom exception hierarchy
- `mediafile/fields.py` - Field descriptor classes (MediaField,
DateField, etc.)
  - `mediafile/storage/` - Format-specific storage strategies
  - `mediafile/utils/` - Utility functions and helpers

##  **Build System & Dependencies**
- **Updated to modern Python packaging** with `pyproject.toml`
- **Dropped Python 3.7 and 3.8 support** - now requires Python ≥3.9
- **Updated dependency specifications** and modernized build
configuration
- **Removed Tox** in favor of plain GitHub Actions for testing
- **New version**: `1.0.0-alpha.1` - reflecting the significant
architectural changes
- **Coverage**: Code test coverage is now uploaded as artifact (maybe we
want to switch to codecov at some point)

Author: semohr

.git-blame-ignore-revs

@@ -1,2 +1,29 @@
-# Migrated to ruff
-7ff9d471b23c45b6d957e7507035af39885546ab
+# Moved changelog into its own file and added changelog reminder.
+7ff9d471b23c45b6d957e7507035af39885546ab
+# Removed flask8 in favor of ruff.
+226e455a5605cf70a4387ba6a17da85d41c6ce87
+
+# Moved mediafile.py into mediafile/__init__.py to allow easier refactoring.
+4d508f94aef6976162993b7d92dc2347120ee306
+# Moved exceptions into own file.
+075749781172208a6ce6bfda0e31e9ab6e42f340
+# Modernized exceptions and added common type.
+891eef5b2d117b07dd74b17fb84e509db9c24f4c
+# Moved constants into own file.
+9470f8e4d7b3bd17372d6c82aa19834f2148a795
+# Moved storage classes into own files and folder.
+146a462e44e0bd4efa4f67a06378686adc8d38b4
+# Moved field classes to own file.
+80145c8c2e0a031c1d4da7a872ca200cc0e2d3af
+# Moved afs unpack util functions to asf module.
+5bfd2cb144ce618c7a8fff7a713addc661bd7465
+# Moved mutagen util functions
+28f336523d26f8ba1d3996c9b90ae5b8c8411b5d
+# Moved soundcheck util functions into own file.
+8a1769f9731cc9aec44c707b07941ce3824e6e0b
+# Moved image util functions into own file
+184f59c727612978e7c5f64ff24e07019094599e
+# Moved safecast util functions.
+a5b5c6c8ff61b0271df36e17633c34503f1de189
+# Fixed imports
+bb3857d297596b589302c9d7b4bd166e5184a59b