AK+LibC: Add TODO() as an alternative to ASSERT_NOT_REACHED()

I've been using this in the new HTML parser and it makes it much easier
to understand the state of unfinished code branches.

TODO() is for places where it's okay to end up but we need to implement
something there.

ASSERT_NOT_REACHED() is for places where it's not okay to end up, and
something has gone wrong.

Libraries/LibWeb/Parser/HTMLTokenizer.cpp

@@ -37,12 +37,6 @@
 #define CONSUME_NEXT_INPUT_CHARACTER \
     current_input_character = next_codepoint();
 
-#define TODO()                                                                                              \
-    do {                                                                                                    \
-        dbg() << "[TODO: " << state_name(m_state) << "] '" << (char)current_input_character.value() << "'"; \
-        ASSERT_NOT_REACHED();                                                                               \
-    } while (0)
-
 #define SWITCH_TO(new_state)              \
     do {                                  \
         will_switch_to(State::new_state); \