v3.1.0

• Modernize build/test/lint tooling:
  • Replace gulp + Babel + esm with tsup, vitest, modern ESLint, and Prettier
• Increase supported Node baseline to >=20
• Add exports and explicit publish files map
• Remove unnecessary runtime dependencies:
  • auto-parse
  • camelcase
  • cross-spawn
• Keep public API compatibility for load, config, parse, preload config entry, and CLI usage
• Expand and modernize test coverage, including CLI execution behavior
• Add layered env file support for path and defaults:
  • Accept string | string[] in code
  • Accept comma-separated values via DOTENV_CONFIG_PATH and DOTENV_CONFIG_DEFAULTS
  • Deterministic merge order with last layer wins
• Add layered schema support with schemaExtends (string | string[] or comma-separated env var)
• Add returnSchemaOnly option to return only schema-defined keys
• Add errorOnMissingFiles option for strict missing-file errors
• Add CLI print mode:
  • --print for JSON output
  • --print=dotenv for KEY=value output
• Add dee CLI alias while preserving dotenv-extended command compatibility
• Expand tests for new config/CLI behavior and maintain full coverage
• Update README documentation for new options and layered-file usage