* chore: vendor Mitt into src/common/third-party
As discussed in #6203 we need to vendor our common dependencies in so
that when we ship an ESM build all imports point to file paths and do
not rely on Node resolution (e.g. a browser does not understand `import
mitt from 'mitt'`).
* chore: Don't store revisions in `package.json`
It's quite messy to have to require the `package.json` file in multiple
places purely to find out what revision of a given browser we want to
use. We can also achieve better type safety by placing it in an actual
source file.
This commit makes that change and also tidies up our reliance on
`package.json` within the source code generally; we now only use it to
find the location of the Puppeteer root such that we know where to
install downloaded browsers to.
To avoid using `package.json` to parse the name of the module, we also
now explicitly have an entry point for the Puppeteer module and the
Puppeter Core module. This will make it easier in the future to ship
less code as part of core (e.g. core never needs to download a browser,
so why ship that code?). Core can also then not have any revisions based
info contained in it.
The test install script has also been updated to ensure that
puppeteer-core can be installed correctly too.
Finally, the `install` script has been moved to TypeScript for nicer
typechecking and safety. The functionality of it has not changed.
