Error links to the error index could open in a new tab

[lqd]

The compiler error codes are turned into links to the error index documentation, however these links open in the same page and not a new tab, making users feel like they just lost their code.

We could add a target="_blank" to those error links. And is somewhat related to #207.

(Of course, they didn't really lose anything as the code is saved in browser storage, but first-time users don't know about this feature until going back to the playground).

This actually happened in real life when I was showing the playground to a new rust user, they clicked the error and gasped in terror :)