Enable lexical binding

[joshbax189]

See https://www.gnu.org/software/emacs/manual/html_node/elisp/Converting-to-Lexical-Binding.html

This blocks #10 (at least my implementation of the feature) and has been the default for a while now, so it's probably a worthwhile improvement.

After a quick look dynamic binding is used in a few places, so some functions may need params added, e.g. js-comint-start-or-switch-to-repl seems to need a way to temporarily set the program arguments NODE_PATH.