Bundle with pack-flat on ci

[etpinard]

Probably not the greatest way to test those unfortunate browser-pack-flat hiccups, but oh well running our image tests with a pack-flat bundle would have been enough to catch #2456. This adds about 1 minute to our total test time on CI unfortunately (browser-pack-flat is kinda slow 🐢 ). I think this is an ok compromise for now. Thoughts?

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Some info from my investigations:

• I didn't find any issues (other than Polar plots broken in v1.35.0 (visible on docs site) #2456.) by flipping through the docs page
• With the current master and a browser-pack-flat bundle, all image tests are passing
• Running the jasmine tests with browser-pack-flat with the current configuration isn't viable. It takes more than 10 minutes to bundle plotly and the test code on CI.
• because of ⤴️ , I couldn't get a full clean run of jasmine test working. I tested a few test suites individually with browser-pack-flat. All test suites I tried were successful.
• Going forward, perhaps using something like exposify so that our jasmine test suites that require lib/index.js grab a pack-flat build/plotly.js bundle would be best.
• ... but really perhaps browser-pack-flat isn't worth the trouble?

[alexcjohnson]

Thanks for all the investigation @etpinard - we can continue discussing pack-flat later, but for now this looks great 💃