feat(core): v0.5.1 - process isolation and syscalls

- Implemented CR3 switching in scheduler for process isolation

- Added 'process' structure and PML4 cloning (vmm_create_address_space)

- Added NASM crt0 and C entry point for userspace apps

kernel/inc/shell/builtins.h

@@ -11,4 +11,5 @@ void print_regs();
 void cmd_sleep();
 void cmd_debug();
 void cmd_rand();
-void cmd_ver();
+void cmd_ver();
+void cmd_userspace();