Add feature to manually load data with button

[Jordan-Ellis]

A common paradigm for infinite scroll is to require the user to click a load more button before infinite scrolling starts. This prevents an issue where somebody can never reach the footer of the website.

This pull requests adds a manualload prop and loadMore slot. When manualload is true, the loadMore slot will be shown which contains a button to load more items. The user has to manually click the button in order to load more, instead of the infinite scroll automatically loading.

This prop has to be manually set which allows for different use cases. For example, within the load function you set manualload to false after the user clicks the button, enabling infinite scroll after the first click. Or you can keep it set to true, essentially turning the component into a load more button that handles all the state.

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