LibCodeComprehension: Re-organize code comprehension related code

This moves all code comprehension-related code to a new library,
LibCodeComprehension.

This also moves some types related to code comprehension tasks (such as
autocomplete, find declaration) out of LibGUI and into
LibCodeComprehension.

Userland/Libraries/LibCodeComprehension/Cpp/Tests/find_variable_declaration.cpp

@@ -0,0 +1,4 @@
+int main(int argc, char** argv)
+{
+    argv = nullptr;
+}