feat(react): expose FallbackRender as top-level type

[MasterOdin]

This PR exposes the FallbackRender type at the root level of @sentry/react package. Currently, to use the type and so get type info on the passed error object., a project I work on does:

import { FallbackRender } from '@sentry/react/types/errorboundary.d';
// v6: import { FallbackRender } from '@sentry/react/dist/errorboundary';

export const ErrorOverlay: FallbackRender = (error) => { /* component code */ });

which feels wrong, as reaching into the package internals is clearly asking for unannounced breaking changes (as seen going from v6->v7).

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[AbhiPrasad]

Thank you for your contribution. This should be out with a release that goes out next week!