core: use dist assets for root export

[djskinner]

For the default export of @bugsnag/core switch to using the dist assets created by rollup/TypeScript. This means all imports of @bugsnag/core will now use the compiled assets rather than the source code. Note that imports such as @bugsnag/core/breadcrumb are still pointing directly at the source code for now

[github-actions]

@bugsnag/browser bundle size diff

	Minified	Minfied + Gzipped
Before	50.02 kB	15.41 kB
After	50.02 kB	15.41 kB
±	No change	No change

code coverage diff

<temporarily disabled>

Generated by 🚫 dangerJS against 4233e12

[djskinner]

In summary I had to change the temporary jest resolver to fix some issues with resolving packageName=@bugsnag/core, submoduleName=. by removing the && submoduleName !== '.' condition. But then needed to return null instead of erroring for node modules that resolve with e.g. packageName=https, submoduleName=.

Hopefully we'll be able to bump jest later and we'll be able to drop this

[djskinner]

@bugsnag/core never had a default export but this is a confusion from mising cjs exports and esm imports. In cjs it would have been:

const { Client, Breadcrumb, Event, Session } = require('@bugsnag/core')` which makes sense against the deleted index file above

[djskinner]

On the question of public versus internal interfaces for the notifiers themselves we can address that later. However I don't think it makes much sense at all for @bugsnag/core to have this concept. We don't document/expect our customers to use @bugsnag/core directly anyway so we can just keep things simple and export the actual interfaces.

For now though, we're still using the manual types - we can't switch to generated types until all the JS files that make up the external interfaces are converted to TypeScript otherwise invalid types are generated.

[djskinner]

This is the main change, the rest of this PR are the consequences