CLC-4650 - Extend scrolling support in canvas-customization.js

https://jira.ets.berkeley.edu/jira/browse/CLC-4650

Extend the `canvas-customization.js` file with scrolling support for the parent container from the BasicLTI iFrame. The following functionalities will be added:

- Allow the BasicLTI iFrame to retrieve the current scroll position of the parent container
- Allow the BasicLTI iFrame to scroll the parent container to a specific position