As per title. (I can't think of a better one)
This would be a useful tool for proof checking the machine/recipes/variables and finding errors in the scripts.
The online JSON formatter is useful, but only for JSON syntax errors- it doesn't cover for issues with incorrectly listed blocks/variables.

This idea is from ContentTweaker's /ct syntax command which lists the filenames and lines where the error lies

I've spend a few hours chasing errors where I've tinkered with a 'variables.var.json' loaded up the game and half the machines have vanished. Or been working on a new machine.json and its suddenly vanished after reloading the game.
(100% of the time its me doing something stupid like using ":" instead of "@" for block meta values)

These error types should be printed into the log during startup already.

?? Such a command exists. And it's basically named like what you wrote: /mm-syntax