Add some more details in todo_list.md for self-hosted compiler

This patch makes it such that the type checker when re-implemented in
the self-hosted compiler for the kernel will check for some common
pitfalls and throw some errors out about them.

Signed-off-by: Slendi <slendi@socopon.com>

todo_list.md

@@ -45,6 +45,12 @@
         - [ ] Lexer
         - [ ] Parser
         - [ ] Type checker
+            - Check if pointers have been checked for NULL before being used,
+              to avoid null dereferencing.
+            - Throw error if a variable is shadowing another one from another
+              scope.
+            - Throw error if a function's return value is not used or
+              explicitly ignored
     - [ ] Re-write the backend
         - [ ] Create an assembler for x86_64
         - [ ] Create a custom backend