* Add `defer` loading strategy for all frontend end-user theme scripts (excluding Customizer preview).
* Move scripts to the `head` which relate to the initial page viewport to ensure they start loading earlier and execute sooner while still not blocking rendering.
* Update Twenty Twenty's script loader (`TwentyTwenty_Script_Loader`) to support core's built-in script loading strategies (#12009), while also retaining backwards-compatibility for child themes that may set `async` and `defer` script data.
* Update the main script loading strategy in Twenty Twenty from `async` to `defer` for better performance on repeat page views, since when an `async` script is cached it will block rendering.
Props westonruter, flixos90, sabernhardt.
Fixes#59316.
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