Feature Request: Delay JavaScript execution

### Impacted plugin

Boost

### What

The ability to delay loading JavaScript files until user interaction

### How

Jetpack Boost doesn't have a feature to delay JavaScript loading until the first user interaction. Similar features exist in plugins like [FlyingPress](https://docs.flyingpress.com/en/article/delay-javascript-y9gvfg/), [WP-Rocket](https://docs.wp-rocket.me/article/1349-delay-javascript-execution), and the stand-alone plugin [Flying Scripts](https://wordpress.org/plugins/flying-scripts/).

When running a Google PageSpeed test, sometimes they would also recommend using one of those plugins for opportunities like `Reduce unused JavaScript` and `Reduce the impact of third-party code` (I tried to replicate that but I couldn't right now, but I distinctly remember seeing them advertising WP Rocket for this feature a few months ago).

This feature would be incredibly valuable to reduce the performance impact of 3rd party code like Google Analytics and the Meta Pixel.

No files, tests, or entry points are named. Start by locating Jetpack Boost's JavaScript-loading entry points and reviewing how third-party scripts are handled; done means eligible JavaScript waits until the first user interaction without breaking site behavior.

Written by the indexing model from the issue text.