repl: show lexically scoped vars in tab completion

Use the V8 inspector protocol, if available, to query the list of
lexically scoped variables (defined with `let`, `const` or `class`).

PR-URL: https://github.com/nodejs/node/pull/16591
Fixes: https://github.com/nodejs/node/issues/983


Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>