Add exec magic command to the REPL

[yotamN]

Sometimes you want to execute a multiline script after you've already opened the REPL and you don't want to close it and lose all state. The exec command will let the user execute a file from the REPL while running, just like typing the code manually.

Again I'm basing this patch on the magics refactors so it won't cause too much conflicts

[oleavr]

Conflicts ⚔️

[yotamN]

Rebased

[oleavr]

Thanks!