Short answer: No. The Fixlet Debugger doesn’t support syntax highlighting in the Action tab, only the Relevance tab has it.
Workaround: paste your script into the Console’s Action tab (create a Custom Action or open any test Task/Fixlet) to see the colorized syntax while you test.
You can also check this thread, I raised a similar ask there. I used that Action Script editor for quite a while, but it eventually stopped working in my environment. Not sure if it still works for others, but worth a look: