[CI] Create an action to run npm run sync

[chalin]

Originally posted by @svrnm in #3948 (comment):

First: +1 for npm run sync fixing 80% of that, I used it the other day and it does a perfect job, maybe we can turn this into a /fix:sync as well?

Great idea to have a /fix:* trigger for npm run sync. Candidate command names:

• /fix:submodules
• /fix:modules
• /sync:submodules -- we don't really have to stick to the fix prefix

Other suggestions are welcome.

[svrnm]

/fix:submodules sounds good to me