Adds browserslist integration for browserify grunt task

[desaikd]

Issue #773 :

Description of changes:

This PR fixes the issue of TypeError: Cannot convert a BigInt value to a number when importing browser bundle of ion-js.

Issue: This is a known issue with babel: babel/babel#13109. (this issues also has a suggested solution)

The fix lies in defining browserslist in package.json for @babel/preset-env. From the babel documentation:

By default @babel/preset-env will use browserslist config sources unless either the targets or ignoreBrowserslistConfig options are set.

Fix: Following the solution specified in babel/babel#13109, added browserslist in package.json. The list of compatible browsers is gathered from AWS SDK supported browsers.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[desaikd]

ion-test-driver seems to fail for its first task of pip install and doesn't seem to be related to this change.