[emacs] Use standard JavaScript mode for now.

This is an attempt to simplify requirements for JavaScript development.

- removes js2 and js3 and packages that depend on them
- switch to v8 as the JavaScript shell
[emacs] Enable markdown mode for appropriate filenames.

This was not require'd and so was not enabled.  It's a little weird that
the standard extensions are not recognized for the mode without adding
them here.