use webpack to generate browser-ready version

[mfrisbey]

Description

Updates the library to use webpack, instead of babel directly, to generate a browser-ready version.

Related Issue

#125

Motivation and Context

Allows usage of the library to upload files in a browser context. This was previously supported, but changes to webpack seem to have broken it.

How Has This Been Tested?

Used in a React application to successfully upload files.

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have signed the Adobe Open Source CLA.
• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.