Now the first pass of migrating to TypeScript is complete I'm going
through the src files one by one to tidy up the public/private
interfaces.
Puppeteer used an underscore convention to denote privacy but violates
this in some places; we should be strict with TypeScript's `public` and
`private` keywords instead.
This means we'll get nice TS errors if you try to refer to a private
method/variable, and means when we swap to generating our TS docs the
tooling knows what method(s) are public and therefore need to be
documented vs private internals that don't.
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