Fix FeatureCollection item validation

[lihouwenbin]

Summary

• require FeatureCollection.features items to be Feature instances during validation
• keep validating nested Feature errors after the item type check
• add regression tests for geometry items and nested FeatureCollections in FeatureCollection.features

Fixes #220.

Validation

• .venv/bin/python -m pytest tests/test_validation.py::TestValidationFeatureCollection
• .venv/bin/python -m pytest
• git diff --check