Add es module build for browser installation

[acmiyaguchi]

Thanks for this great package! I've had fun using it so far, and it's one of the few packages in the ecosystem that fits my needs.

I'm using a browser bundler for my application, so the current set of builds are problematic. When I use npm install midi-player-js for my web app, it also includes the require("fs") statement in loadFile. I did a couple things to resolve this:

• Conditionally added a section for loadFile that rollup eliminates as a dead branch when being built for the browser
• Add a es module for browser distribution under builds/index.browser.js. I add this to the package.json under browser so my bundler grabs the correct distribution.

I also removed the build directory from check ins. I left the browser directory alone, in case the github raw link was being used to source the script.

This works as expected in a small test page I made.

[grimmdude]

Wow thanks @acmiyaguchi! This all looks great, truly appreciate your contribution. I know that require('fs') has been a thorn in some people's sides for a while.

Agree that docs should be handled differently so they're not always mixed up with code changes.

[vinzentt]

I'm still having he issue with fs module not found because this PR was merged to master without a release. Do you know when the next release is planned?

[grimmdude]

Hey @vinzentt, thanks for the reminder. I just published 2.0.14 to npm which includes this change.