feat: add support for on new session start callback

[kevinpagtakhan]

Summary

This PR adds support for on new session start callback. Callback functions can be registered to onNewSessionStart event using the following ways:

Method 1

amplitude.getInstance().onNewSessionStart(() => {
  console.log('new session - method 1');
});

Method 2

amplitude.init('API_KEY', USER_ID, {
  onNewSessionStart: () => {
    console.log('new session - method 2')
  },
});

Testing

• Unit tests are passing
• Manually tested by executing the following below:

// cookies are deleted and page is refreshed

// register onInit callback
> amplitude.getInstance().onInit(() => console.log('init'))

// register session callback 1
> amplitude.getInstance().onNewSessionStart(() => console.log('session - method 1'))

// register session callback 2
// invoke init - runs registered callback fns
> amplitude.init('a2dbce0e18dfe5f8e74493843ff5c053', undefined, { onNewSessionStart: () => console.log('session - method 2') })
init
session - method 1
session - method 2

// invoke init - does not run registered callbacks fns
// init callback is already reset
// session callbacks are not called because no new session is created
> amplitude.init('a2dbce0e18dfe5f8e74493843ff5c053')

// invoke setUserId and create new session  - runs registered callback fns
> amplitude.getInstance().setUserId('asdf', true)
session - method 1
session - method 2

Checklist

• Does your PR title have the correct title format?
• Does your PR have a breaking change? None

Closes #429

[jooohhn]

LGTM!