Add extensions to import statements

[phaux]

...so that the library can be consumed natively using JS modules

export { symbols } from './src/symbols';
export { notifications } from './src/notifications';
export { importNode } from './src/node_data';
// should become
export { symbols } from './src/symbols.js';
export { notifications } from './src/notifications.js';
export { importNode } from './src/node_data.js';