fix(types): disambiguate SanityClient imports

[stipsan]

In the JS world these two are the same instance:

import {SanityClient} from '@sanity/client'
import {SanityClient as SanityCoreClient} from '@sanity/client/stega'

assert(SanityClient === SanityCoreClient) // all good

But due to how we generate typings, TypeScript thinks they're not compatible as they're classes with private properties:

import type {SanityClient, SanityStegaClient} from '@sanity/client/stega'

interface Props {
  client: SanityClient | SanityStegaClient
}
function Component(props: Props) {}

// Later on
import {createClient} from '@sanity/client'
<Component client={createClient()} />

But now it'll throw an error like:

Argument of type 'import("@sanity/client/dist/index").SanityClient' is not assignable to parameter of type 'import("@sanity/client/dist/stega").SanityClient'.
  Property '#private' in type 'SanityClient' refers to a different member that cannot be accessed from within type 'SanityClient'.ts(2345)

Errors are usually not as clear as the above btw, it took me a while to figure out what was wrong when I first encountered this. The fix is not obvious:

-import type {SanityClient, SanityStegaClient} from '@sanity/client/stega'
+import type {SanityClient} from '@sanity/client'
+import type {SanityStegaClient} from '@sanity/client/stega'

So the point of this PR is that if you now do this:

import type {SanityClient} from '@sanity/client'

You'll get a clear error:

Argument of type 'SanityStegaClient' is not assignable to parameter of type 'never'.ts(2345)

And the import uses a @deprecated symbol to make sure VS Code don't autocomplete it, and clearly highlights it: