Add JSON schema for symbol config

[zhouzhou626]

Summary

• Add a draft 2020-12 JSON schema for symbol.toml / [tool.symbol] config.
• Update the README configuration example to match the current parser shape (checker, checkers.*, etc.).
• Add schema tests covering JSON validity, key config sections, and the documented config shape.

Closes #5.

Tests

• uv run pytest tests\test_config_schema.py tests\test_project_config.py
• uv run ruff check tests\test_config_schema.py
• python -m json.tool schemas\symbol.config.schema.json > $null
• uv run python -c "import json; from pathlib import Path; from jsonschema import Draft202012Validator; Draft202012Validator.check_schema(json.loads(Path('schemas/symbol.config.schema.json').read_text(encoding='utf-8')))"
• uv run pytest on Windows: 332 passed, 22 failed. The failures appear unrelated to this PR and come from existing Windows platform assumptions around CRLF byte offsets, path separators, and chmod behavior.

[aaliboyev]

Thanks — verified the schema against project_config.py and config_resolver.py, all accurate. Will give it one more pass and merge.