AK+Everywhere: Consolidate String::index_of() and String::find()

We had two functions for doing mostly the same thing. Combine both
of them into String::find() and use that everywhere.

Also add some tests to cover basic behavior.

Userland/Utilities/expr.cpp

@@ -371,7 +371,7 @@ private:
         }
 
         if (m_op == StringOperation::Index) {
-            if (auto idx = m_str->string().index_of(m_pos_or_chars->string()); idx.has_value())
+            if (auto idx = m_str->string().find(m_pos_or_chars->string()); idx.has_value())
                 return idx.value() + 1;
             return 0;
         }