Fix typings for 'never' server context

[stazz]

The file server/src/flow.ts has type GetContext which now turns into never when server context is never. This needs updating.

Alternatively, maybe switch from never server context into something else like undefined? The never type has caused already a bit of a havoc after introducing it, as it doesn't always behave as one would expect (e.g. X extends never ? A : B doesn't work like expected, etc).

[stazz]

The createTypicalServerFlow actually might need overloading in any case, since returned callback should not take any arguments.

[stazz]

Might as well also expose the version of createTypicalServerFlow which takes squashed endpoint as argument, so that reconstructing the large RegExp can be avoided e.g. on every lambda startup