add node build

a nodejs build is located in dist/index.js

Example use:

$ node
Welcome to Node.js v14.17.4.
Type ".help" for more information.
> const hdf5 = require("jsfive")
undefined
> var fs = require("fs");
undefined
> var ab = fs.readFileSync("/home/brian/Downloads/sans4490.nxs.ngv");
undefined
> var f = new hdf5.File(ab.buffer);
undefined
> f.keys
[ 'entry' ]
> f.get("entry").attrs
{ NX_class: 'NXentry' }
>