fix(ts): match `MiddlewareConfig` with documentation (#61718)
### What?
Fix the user-facing `MiddlewareConfig` interface.
~While in the codebase, I also made the incoming config object type a
bit more strict by converting from `any` to `unknown`.~ Reverted, as we
do a config assertion already in a [different
place](https://github.com/vercel/next.js/blob/canary/packages/next-swc/crates/next-custom-transforms/src/transforms/page_config.rs/#L171-L180).
### Why?
The interface we previously exposed was the one we used internally,
_after_ we did some parsing on the config object, which is different
from what the user is expected to pass.
### How?
I separated the internal type to its own `MiddlewareConfigParsed`
interface.
Closes NEXT-2375
Fixes #61705
Ref: #61576
