Add proper types to next modalfold version

[jakublala]

Add type hints to modalfold package

Currently mypy reports missing type stubs for modalfold. To fix this and improve type safety:

1. Add inline type hints to all relevant files (e.g., esmfold.py)
2. Create empty py.typed marker file in the package root
3. Update pyproject.toml to include the marker:

[project]
include = ["py.typed"]

This will enable proper type checking in projects that depend on modalfold.