Switching to rnodejs package to wrap node

This PR removes node.R and imports rnodejs, which provides expanded functionality.

-   It includes the `dat` module as part of the r package under `inst/node`
-   It changes `dat_init` to the rnodejs API, (e.g., dat_init_internal = node_fn("dat", "init"))
-   It adds checks for node.js and dat dependencies to `.onAttach`