[BUGIFX] Fix path to JS debug script

Use absolute URL to point to the JS debugger library.

Resolves: #58677
Releases: 1.6
Change-Id: Ic68f55801d76fdf45be7fd02f819c9fe0725b864
Reviewed-on: https://review.typo3.org/29975
Reviewed-by: Francois Suter
Tested-by: Francois Suter