Adds entrypoint

[spacenate]

As a developer building a GAS project using ES modules (import/export), I would like to be able to import this package:

import { OAuth2 } from 'OAuth2';

Currently, this fails with an error like so:

Module not found: Error: Can't resolve 'OAuth2' in '/Users/nathan/.../src'

Field 'browser' doesn't contain a valid alias configuration

After declaring an entry-point for this package (via package.json), then I am able to import the package without error.

[google-cla]

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