add create-browser-app cli to stagehand

[kamath]

why

We can use create-browser-app to create a stagehand app. Want to add this here for consistency. Will likely need another PR where we move Stagehand-specific code into a stagehand/ or src/ directory; this is getting pretty messy.

what changed

nothing to users. just added source for create-browser-app cli

test plan

works on npm

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Latest commit: 74050b3

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[kamath]

closing, doesn't need to be in this repo