fix: Target window.top.document.body; remove code for adjusting viewport changes

[pbt]

There are two main changes here:

1. Change the target to document.body ➡️ window.top.document.body. This is because in certain situations we might want to invoke the body scroll lock while inside of an iframe and want to make sure locking extends beyond the iframe.
2. Remove the code that adjusts for viewport changes. While debugging this library I found that this passage of code that attempts to adjust for viewport changes did not work (it was causing the top value to be unset, meaning scroll position was not preserved). I don't think that it ought to be the responsibility of this library to adjust for viewport changes; it should just be to apply a body scroll lock. If they need to, consumers can adjust by listening to changes in visualViewport or using the new CSS display viewport dimensions.

[emeraldsanto]

Thanks for the detailed description 😌