[vitest-pool-workers] Expose assets in exports.default.fetch()

[NonSpicyBurrito]

In 0.13.0, SELF.fetch() was removed and replaced with exports.default.fetch(), which behaves identically except that assets are not exposed. Cloudflare recommends testing assets using a different strategy such as startDevWorker().

However, whether a particular URL is implemented in worker or in assets is an implementation detail and should be irrelevant to the test. The test should not need to know this to test a particular URL. A common scenario is that the response of a URL has become static, and can be refactored to use assets as cost and performance optimization; in this case the refactor should not break its test, but it does.

I would like to request assets to be exposed in exports.default.fetch().