Split the first-time installation into two phases

This allows the shell environment to be properly prepared for systems like rbenv and nodenv to work.
Use OiNutter’s actively maintained nodenv package

Use this one instead of wfarr’s, which hasn’t been touched in over a year. This change also fixes a problem where the `npm` binary wasn’t visible immediately after selecting a global node version – it required a new shell to be opened. This problem doesn’t exist in OiNutter’s version of nodenv.

This is the last fix needed for a 100% smooth and error-free first run of babushka.
Tidy up the naming and structure of the node deps

This brings them into line with how we’re handling the Ruby ones.
Tidy up the ruby deps

Remove some comments and todos.