* chore: remove src/externs.d.ts
It defined global types that we don't want to use, and instead we move
to using interfaces that we import and reference just like with any
other interface.
This means other than Protocol (which I think is fine to leave as is),
there are no other magic global types and you have to import any types
or interfaces that you want.
* chore: migrate src/Page.js to TypeScript
The final one! This is a huge file and needs to be split up and tidied,
but for now I've left all the definitions in place and converted types
accordingly.
There's some additional tidying we can do now every `src` file is TS,
but I'll leave that for another PR to avoid this one getting any bigger.
Co-authored-by: Mathias Bynens <mathias@qiwi.be>
