Added path::is_link support

[ssenthilnathan3]

Closes #3448.

• Uses lstat on Linux, Android, Darwin, BSD, and Emscripten.
• Uses FILE_ATTRIBUTE_REPARSE_POINT on Windows.
• Adds the required platform-specific libc lstat bindings.
• Returns false for missing paths, consistent with path::is_file and path::is_dir.
• Detects dangling symbolic links on POSIX platforms.

[lerno]

Thank you!