Improve invalid template config errors

[Vinayak1337]

Summary

• add a shared loader for template conf.json files
• report the template configuration file path when JSON parsing fails
• add a regression test for invalid template config JSON

Why

When a template conf.json is invalid, json.load() raises a JSONDecodeError without showing which template file caused the failure. That is hard to diagnose when multiple template paths are configured.

Fixes #2149.

Testing

• .venv/bin/python -m pytest -q tests/exporters/test_templateexporter.py::TestExporter::test_invalid_template_conf_json_error_includes_path
• .venv/bin/python -m pytest -q tests/exporters/test_templateexporter.py
• .venv/bin/python -m ruff check --fix --show-fixes nbconvert/exporters/templateexporter.py tests/exporters/test_templateexporter.py
• .venv/bin/python -m ruff format nbconvert/exporters/templateexporter.py tests/exporters/test_templateexporter.py
• git diff --check
• manual smoke test confirming the raised error includes the invalid conf.json path