LibCore: Add File::{stdin, stdout, stderr}()

This should make it easier to get a Core::File for standard streams.

Libraries/LibCore/File.cpp

@@ -203,4 +203,34 @@ String File::read_link(const StringView& link_path)
 
 #endif
 
+static RefPtr<File> stdin_file;
+static RefPtr<File> stdout_file;
+static RefPtr<File> stderr_file;
+
+NonnullRefPtr<File> File::stdin()
+{
+    if (!stdin_file) {
+        stdin_file = File::construct();
+        stdin_file->open(STDIN_FILENO, IODevice::ReadOnly, ShouldCloseFileDescription::No);
+    }
+    return *stdin_file;
+}
+
+NonnullRefPtr<File> File::stdout()
+{
+    if (!stdout_file) {
+        stdout_file = File::construct();
+        stdout_file->open(STDOUT_FILENO, IODevice::WriteOnly, ShouldCloseFileDescription::No);
+    }
+    return *stdout_file;
+}
+
+NonnullRefPtr<File> File::stderr()
+{
+    if (!stderr_file) {
+        stderr_file = File::construct();
+        stderr_file->open(STDERR_FILENO, IODevice::WriteOnly, ShouldCloseFileDescription::No);
+    }
+    return *stderr_file;
+}
 }

Libraries/LibCore/File.h

@@ -57,6 +57,10 @@ public:
     };
     bool open(int fd, IODevice::OpenMode, ShouldCloseFileDescription);
 
+    static NonnullRefPtr<File> stdin();
+    static NonnullRefPtr<File> stdout();
+    static NonnullRefPtr<File> stderr();
+
 private:
     File(Object* parent = nullptr)
         : IODevice(parent)