Feature Request: Update the extension's default prehiding script to hide the body

### Expected Behaviour
The prehiding script functionality would match the precedent set by Target and Launch previously, which was to prehide the full tag.

### Actual Behaviour
The prehiding script that is available via the Web SDK extension hides a custom **personalization-container** class which would need to be added to each element that will be personalized. If a customer did not know this change was made in the prehiding script, then they would see content flicker.

### Reproduce Scenario (including but not limited to)

#### Steps to Reproduce

#### Platform and Version

#### Sample Code that illustrates the problem

#### Logs taken while reproducing problem

Start by locating the Web SDK extension's default prehiding script and review how Target and Launch previously handled prehiding. The change is complete when the default script hides the full rather than requiring a personalization-container class; add or update coverage if the relevant tests are found nearby.

Written by the indexing model from the issue text.