Convert scripts/Gulpfile to checked mjs/cjs so they can run without compilation

[Jake Bailey (jakebailey)]

This overall makes the build and branch switches faster. It's also a prerequisite for sharing some code between scripts and the scripts/built modules (which are only used from the gulpfile).

To prevent any annoying gitignore troubles, all files are either mjs or cjs, any .js files are ignored so going forward/back in the commit history shouldn't leave any annoying files around.

I had to update node-fetch; the previous version hits the new-ish issues with 4.8's default behavior. I don't think this has any major impact on daily use of the repo, so differences in the versions shouldn't cause annoyances when bisecting.

Leaving this as a draft until I go around to figure out what all are using these scripts directly, e.g. open-cherry-pick-pr and such are not referenced in this repo so probably are defined in some build somewhere.

Things that have to change external to this PR:

• There are various pipelines on AzDo that directly compile scripts and then run them. They can be fixed pretty easily by checking if [ -f "./scripts/open-cherry-pick-pr.mjs" ]; then ... else ... fi.
• We have a PR task which screws around with chalk; this should be a no-op at this point given we now pin a specific verison, but when we're mjs, it's never a problem since the whole reason for the fix is to get around ESM stuff anyway.