Add support for loading multiple .env files sequentially

[Copilot]

Users could only specify a single .env file path. This adds support for arrays of paths loaded in order, with later files overriding keys from earlier ones while preserving unique keys from all files.

Changes

• ConfigurationOptions — new absolutePaths?: string[] and relativePaths?: string[] options alongside the existing single-path options
• resolveOptionsFilepaths — new function (exported from index) that expands all path options (single + array) into an ordered PathLike[]
• configure — iterates all resolved paths, merging parsed key-values via Object.assign so later files win on conflicts

Usage

const env = configure(AppEnvironment, {
  relativePaths: [
    '.env',
    '.env.local',
    `.env.${process.env.NODE_ENV}`,
  ],
});