feat(file): add mode and dirMode options for file/directory permissions

[mixelburg]

Adds support for configurable file and directory permissions in the File transport.

Changes

• mode option: Sets the file permission mode (e.g. 0o640) passed to fs.createWriteStream. Defaults to Node.js default (0o666) when not specified.
• dirMode option: Sets the directory permission mode (e.g. 0o750) used when creating log directories with fs.mkdirSync. Defaults to Node.js default when not specified.
• Updated TypeScript definitions for both options.
• Added unit tests.

Usage

const logger = winston.createLogger({
  transports: [
    new winston.transports.File({
      filename: 'app.log',
      dirname: '/var/log/myapp',
      mode: 0o640,
      dirMode: 0o750
    })
  ]
});

Fixes #2026