Finalize next-dev's DevBindingsOptions API (and remove __experimental__ prefix)

[dario-piotrowicz]

In #486 we've added the next-dev submodule for adding local bindings to next dev.

The submodule exports the setupDevBindings function which accepts a DevBindingsOptions object for the various options.

Most of the options there follow Miniflare's API with the exception of textBindings (which in Miniflare is just bindings), adding a new term there could be worse/more confusing then just sticking with the (potentially confusing) bindings term (see #486 (comment)).

So before our next release we need to decide how the DevBindingsOptions object needs to look like, the alternatives are:

• leave it as is (as it is clear enough)
• leave it as is but replace textBindings with bindings (to align it more with Miniflare)
• come up with some new structure that could make things more clear/intuitive (for example passing the bindings in an object structured like)

[dario-piotrowicz]

I had to make a release so that we could get this hot-fix in: #534

So I did release the next-dev submodule but I marked it as experimental: #537

So now what needs to be done is to revert #537 when we finalize the next-dev submodule's API

[dario-piotrowicz]

I think we should follow Wrangler's new API here

[dario-piotrowicz]

TODO after changing the API:

• update the docs accordingly
• in the docs add examples of usage of the supported supported resources (add env vars, DO, R2, etc...)
  (they could live in the examples documentation and have the next-dev docs link to them maybe)

[dario-piotrowicz]

Note: the new Wrangler API accepts Uint8Array types for vars but Miniflare does not yet (but it's going to)

[dario-piotrowicz]

I've added the blocked by external label as this requires wrangler to be properly updated to surface the Bindings type (and potentially the getBindingsProxy utility) before we can proceed with this