Rethink public API

[frostney]

Currently, the API is based upon the change-case libraries.
I think changing the API from changeCaseObject.camelCase(obj) to changeCaseObject(obj).toCamelCase() feels more in-line with native JavaScript methods such as toString, toLowerCase, toUpperCase, etc.

[stevenvachon]

@frostney I think it's better to keep it as is, so that we can do this:

import {camelCase} from 'change-case-object';
camelCase(obj);

const {camelCase} = require('change-case-object');
camelCase(obj);

[MaffooBristol]

I think it'd be nice to be able to do either. I initially assumed it'd be cco(obj).toFoo() too, had to check the docs to get the correct form.