Add embedded nREPL console

[avli]

Currently, the only way to interact with the embedded nREPL is connecting to it with the lein reel :connect <port> command. It would be nice if the nREPL console is available right after the start.

[avli]

After reading recent feedback on the VSCode Marketplace, I think this feature should be implemented as follows:

1. There should be a "Clojure: Open REPL" action available through the command palette which opens a terminal with a running Clojure REPL.
2. There should be an option that always opens a terminal with the REPL as soon as the extension connects.