fix(types): allow null as fetcher in PublicConfiguration - #4290

What

Adds | null to PublicConfiguration.fetcher so passing null as a fetcher via the config object is accepted by TypeScript.

Why

Passing null as fetcher is a valid runtime pattern to disable fetching for a key. The positional overloads on SWRHook already typed fetcher as Fetcher | null, so useSWR("key", null) worked fine. But PublicConfiguration.fetcher was typed as just Fn, causing type errors for the config-object pattern:

useSWR("key", { fetcher: null })          // ❌ TS error before fix
const cfg: SWRConfiguration = { fetcher: null }  // ❌ TS error before fix
useSWR("key", null)                        // ✅ already worked

Fix

- fetcher?: Fn
+ fetcher?: Fn | null

Also adds a type test covering both patterns to prevent regression.

All 365 unit tests pass. Full tsc --noEmit passes.

Fixes #2888