Add Exists function to pathx module

[piraz]

Implement the Exists function in the pathx module to provide a reliable way to check if a given path (file or directory) exists. This should serve as a convenience utility for path presence checks, simplifying file and directory verification.

Acceptance criteria:

• The function signature must be: Exists(path string) bool
• The function returns true if the specified path (file or directory) exists, false otherwise
• Include English GoDoc comments
• Add unit tests for the Exists function in the pathx package
• Code must follow project standards for ioook and Firenado

Example usage:

if pathx.Exists("/etc/passwd") {
    // do something
}