I've tried the Intellij actions plugin, which supports only rudimentary yaml and some general yaml section support.
The vsc actions plugin looks like it may work, as it attempts to format the embedded JS as JS, but not when syntactically correct for an actions workflow.
eg:

The above refuses to apply any syntax/formatting to the JS block within the action step. (With the exception of the "${{ ... }}" blocks)

Adding an empty line afterscript: | shows additional formatting, but now the script syntax is incorrect from the actions perspective:

Given the ridiculously complicated scoping/syntax/context rules within actions, something like this, that reliably works, and has some GH actions variable/global knowledge is desperately needed.

I expect I'm just doing it wrong.

Any pointers on this greatly appreciated.