Kernel: Implement shebang executables ("#!/bin/sh")

This patch makes it possible to *run* text files that start with the
characters "#!" followed by an interpreter.

I've tested this with both the Serenity built-in shell and the Bash
shell, and it works as expected. :^)

Kernel/Process.h

@@ -310,6 +310,8 @@ private:
     int alloc_fd(int first_candidate_fd = 0);
     void disown_all_shared_buffers();
 
+    KResultOr<String> find_shebang_interpreter_for_executable(const String& executable_path);
+
     Thread* m_main_thread { nullptr };
 
     RefPtr<PageDirectory> m_page_directory;