LibJS+js: Support getting last value from "_" variable

The interpreter now has an "underscore is last value" flag, which makes
Interpreter::get_variable() return the last value if:

- The m_underscore_is_last_value flag is enabled
- The name of the variable lookup is "_"
- The result of that lookup is an empty value

That means "_" can still be used as a regular variable and will stop
doing its magic once anything is assigned to it.

Example REPL session:

> 1
1
> _ + _
2
> _ + _
4
> _ = "foo"
"foo"
> 1
1
> _
"foo"
> delete _
true
> 1
1
> _
1
>

Userland/js.cpp

@@ -575,6 +575,7 @@ int main(int argc, char** argv)
         ReplConsoleClient console_client(interpreter->console());
         interpreter->console().set_client(console_client);
         interpreter->heap().set_should_collect_on_every_allocation(gc_on_every_allocation);
+        interpreter->set_underscore_is_last_value(true);
         if (test_mode)
             enable_test_mode(*interpreter);