Add support for retrieving current process user ID (UID)

[samujjal-gogoi]

Currently, we have the pid property in the dart:io library, which allows retrieving the current process ID. However, there is no built-in method or property to fetch the User ID (UID) of the current user running the Dart program.

Adding support for a uid property or method would make it easier to retrieve the current user's UID directly in Dart without relying on platform-specific code or external commands.

[dart-github-bot]

Summary: Users request a dart:io addition: a uid property or method to retrieve the current process's user ID (UID). This avoids platform-specific workarounds.