feat(handler): Server and environment agnostic handler (enisdenjo#37)

BREAKING CHANGE: The handler is now server agnostic and can run _anywhere_

- Core of `graphql-sse` is now server agnostic and as such offers a handler that implements a generic request/response model
- Handler does not await for whole operation to complete anymore. Only the processing part (parsing, validating and executing)
- GraphQL context is now typed
- Hook arguments have been changed, they're not providing the Node native req/res anymore - they instead provide the generic request/response
- `onSubscribe` hook can now return an execution result too (useful for caching for example)
- Throwing in `onNext` and `onComplete` hooks will bubble the error to the returned iterator

### Migration

Even though the core of graphql-sse is now completely server agnostic, there are adapters to ease the integration with existing solutions. Migrating is actually not a headache!

Beware that the adapters **don't** handle internal errors, it's your responsibility to take care of that and behave accordingly.

#### [`http`](https://nodejs.org/api/http.html)

```diff
import http from 'http';
- import { createHandler } from 'graphql-sse';
+ import { createHandler } from 'graphql-sse/lib/use/http';

// Create the GraphQL over SSE handler
const handler = createHandler({ schema });

// Create an HTTP server using the handler on `/graphql/stream`
const server = http.createServer((req, res) => {
  if (req.url.startsWith('/graphql/stream')) {
    return handler(req, res);
  }
  res.writeHead(404).end();
});

server.listen(4000);
console.log('Listening to port 4000');
```

#### [`http2`](https://nodejs.org/api/http2.html)

```diff
import fs from 'fs';
import http2 from 'http2';
- import { createHandler } from 'graphql-sse';
+ import { createHandler } from 'graphql-sse/lib/use/http2';

// Create the GraphQL over SSE handler
const handler = createHandler({ schema });

// Create an HTTP server using the handler on `/graphql/stream`
const server = http.createServer((req, res) => {
  if (req.url.startsWith('/graphql/stream')) {
    return handler(req, res);
  }
  res.writeHead(404).end();
});

server.listen(4000);
console.log('Listening to port 4000');
```

#### [`express`](https://expressjs.com/)

```diff
import express from 'express'; // yarn add express
- import { createHandler } from 'graphql-sse';
+ import { createHandler } from 'graphql-sse/lib/use/express';

// Create the GraphQL over SSE handler
const handler = createHandler({ schema });

// Create an express app
const app = express();

// Serve all methods on `/graphql/stream`
app.use('/graphql/stream', handler);

server.listen(4000);
console.log('Listening to port 4000');
```

#### [`fastify`](https://www.fastify.io/)

```diff
import Fastify from 'fastify'; // yarn add fastify
- import { createHandler } from 'graphql-sse';
+ import { createHandler } from 'graphql-sse/lib/use/fastify';

// Create the GraphQL over SSE handler
const handler = createHandler({ schema });

// Create a fastify app
const fastify = Fastify();

// Serve all methods on `/graphql/stream`
fastify.all('/graphql/stream', handler);

fastify.listen({ port: 4000 });
console.log('Listening to port 4000');
```

.github/workflows/continuous-integration.yml

@@ -59,4 +59,5 @@ jobs:
       - name: Install
         run: yarn install --immutable
       - name: Test
-        run: yarn test --forceExit
+        timeout-minutes: 1 # detectOpenHandles will probably hang if leaking
+        run: yarn test --detectOpenHandles