Fix message lookup and support create-react-app

A bug was introduced when adding the BTree-V2 objects parsing: messages are read and stored with offsets relative to the global file buffer (unlike in pyfive) but the message-parsing functions were using addresses relative to the start of the message block (which is not collated in jsfive), and has been fixed.

Support for create-react-app has been added by pre-compiling the libraries with transforms for the class fields/properties. To use jsfive in this context you can npm i jsfive and then in App.js use

import * as hdf5 from 'jsfive/dist/index.js';