feat: Experimental --discriminator and --require-discriminator

[eliassjogreen]

See #10

Still very experimental.

Allows you to generate fetch types which look like the following:

// With --experimental-discriminator=api
function fetch<T extends "api" = "api">(input: `/api/v1/users/login`);

// With --experimental-discriminator=api --experimental-require-discriminator
function fetch<T extends "api">(input: `/api/v1/users/login`);

This is also helpful for projects which are using multiple different backends as a way of namespacing the types for each:

// With --experimental-discriminator=imdb --experimental-require-discriminator
function fetch<T extends "imdb">(input: `/api/v1/movies`);

// With --experimental-discriminator=api --experimental-require-discriminator
function fetch<T extends "api">(input: `/api/v1/users/login`);

---

To use the fetch with a discriminator specified you simply need to pass it as the first generic argument:

// With --experimental-discriminator=api --experimental-require-discriminator
function fetch<T extends "api">(input: `/api/v1/users/login`);

// Works!
fetch<"api">("/api/v1/users/login", { ... });

// Fails!
fetch<"imdb">("/api/v1/users/login", { ... });

// Falls back to the default `fetch` type which isn't going to be typed.
// When not using the `--experimental-require-discriminator` flag this
// will still work as before. E.g. returning the types if any are found
// matching the function signature.
fetch("/api/v1/users/login", { ... });