core(replay): @puppeteer/replay stringify extension

[adamraine]

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@puppeteer/replay stringify will take a user flow JSON as input, and output a standalone Puppeteer script. We can use the stringify extension api to inject Lighthouse user flow code into the output script.

The intent is for this to be rolled into DevTools, and used to add an "Export as Puppeteer script w/ Lighthouse" option to the list of export options. We could also support users importing this extension themselves, but that is not 100% necessary at this time.