Allow bundle to have multiple entries

[samijaber]

We would like to export internals for our own usage under unsplash-js/internals instead of polluting the main unsplash-js scope and increasing bundle size for all consumers.

This should be doable in rollup, but we're using tsdx which has a bug around this feature: jaredpalmer/tsdx#175

See #149 for more details.

[OliverJAsh]

If the consumer is using tree shaking then the new internals module shouldn't pollute the bundle size, as long as they don't import anything from there.