feat: add followSymlinks option to restrict symlink traversal

[DebuggingMax]

Summary

This PR adds a new followSymlinks option that allows restricting symlink traversal to stay within the configured root directory.

Motivation

Currently, send resolves file paths using string normalization but does not check whether the final canonical path (after symlink resolution) remains inside the configured root. This means symbolic links inside the root can point to — and serve — files outside of it.

This is especially problematic for applications that serve user-writable directories where users could create symlinks pointing to sensitive system files.

Implementation

When followSymlinks is set to false:

• Uses fs.realpath() to resolve the canonical path of requested files
• Rejects the request with 403 if the resolved path falls outside the root directory
• Works with all file serving methods (direct files, extensions fallback, index files)

Options

• followSymlinks: true (default) - Current behavior, symlinks are followed without restriction
• followSymlinks: false - Reject requests for symlinks that resolve outside root

Requirements

• The root option must be set for the symlink check to apply
• This is a semver-minor change with no breaking changes

Example

const send = require("send");

// Reject symlinks pointing outside /www/public
send(req, path, { 
  root: "/www/public",
  followSymlinks: false
}).pipe(res);

Changes

• Added followSymlinks option to SendStream constructor
• Added checkSymlink method to verify paths remain within root
• Updated sendFile and sendIndex to check symlinks when option is disabled
• Added documentation in README.md
• Added comprehensive test suite with symlink fixtures

Tests

All 146 tests pass, including 7 new tests for the followSymlinks option:

• Default behavior (follow symlinks)
• Allow internal symlinks when disabled
• Reject external symlinks when disabled
• Regular file serving when disabled
• Integration with extensions option
• Integration with index files
• Behavior without root option

Closes #297