rename __fetch_polyfill to installFetch

[Rich-Harris]

Describe the problem

I have no idea why it's called that.

Describe the proposed solution

import { installFetch } from '@sveltejs/kit/install-fetch';

Alternatives considered

No response

Importance

nice to have

Additional Information

No response

[Conduitry]

I think that name came from before we were bundling things the same way we are now, and we wanted a name that was unlikely to collide with user code.

[Rich-Harris]

Also, I don't think there's a good reason for that module to re-export fetch, Headers, Request and Response