LibWeb: Prepare script when src is set the first time

From https://html.spec.whatwg.org/multipage/scripting.html#script-processing-model:
When a script element el that is not parser-inserted experiences one
of the events listed in the following list, the user agent must
immediately prepare the script element el:
- [...]
- The script element is connected and has a src attribute set where
  previously the element had no such attribute.

Tests/LibWeb/Text/input/script-src-set-after-insertion.js

@@ -0,0 +1 @@
+println("PASS");