ref(nextjs): Update vercel testing script

[lobsterkatie]

This is a small refactor to our set-up-branch-for-test-app-use.sh vercel script, which prepares the current branch to be tested on vercel by deleting packages unrelated to the nextjs SDK, thereby speeding up the vercel build.

The only consequential change is a switch from specifying which packages should get deleted to specifying which packages shouldn't. (The list of deletion targets kept falling out of date, each time we added a new package. With this change, it should in theory be evergreen.)

[lobsterkatie]

Gah. Why does the auto-formatter insist on indenting these comments? 😖

[AbhiPrasad]

While we are here, can we just convert this to a node script?

[lobsterkatie]

While we are here, can we just convert this to a node script?

If I get to it in the next few days, sure, but otherwise I think I'll merge this just to get it in, and then we can look at converting all of the vercel scripts separately.

[AbhiPrasad]

Cool, sounds good to me

[lobsterkatie]

While we are here, can we just convert this to a node script?

If I get to it in the next few days, sure, but otherwise I think I'll merge this just to get it in, and then we can look at converting all of the vercel scripts separately.

Cool, sounds good to me

Haven't gotten a chance to do this, so I'm going to merge this in and come back to the conversion question in a different PR.