[Performance] Fast path to resolve body selector

[Telroshan]

It's a pretty common pattern to define trigger events using the from:body syntax.

The issue: this "body" selector currently results in a call to document.querySelectorAll inside querySelectorAllExt

While this may seem a negligible thing, it turns out that on large HTML pages, this makes the initial page load & parsing way slower.

• In this JSFiddle, using the current state of the lib (open the developer console to see the logged execution times), I get in average the following:
  querySelectorAllExt called 5000 times with 'body', took 180 ms
• In this JSFiddle, using the suggested change, I get in average the following:
  querySelectorAllExt called 5000 times with 'body', took 3.5 ms

The example code here is simply letting htmx initialize 5 000 divs, that have the exact same hx-trigger specified, which is in this example hx-trigger="customEvent from:body"

While it is negligible on a typical HTML page where you wouldn't have thousands of divs, this example isn't taken out of nowhere, it's actually a real use case I have on Zorro, where a board could totally have thousands of tasks at once.
I'm using a similar hx-trigger specifier for every task to trigger individual refreshes on some actions, such as hx-trigger="refreshTask{{ .ID }} from:body".

The suggested change here helps cutting down initial page load times, which contributes to user experience.
The cost is simply 2 more lines of code in the core.

[alexpetros]

Great 👍