Switch to import = for ws types, since @types/ws uses export =.

While it would be perfectly fine to use `import *` in this particular case,
we've moved away from the star-import pattern in this repository, instead
preferring the `esModuleInterop` compiler option which seems to be the
direction that TypeScript is moving as it's now the default with `tsc --init`.

TypeScript clearly lost its battle with `import *` which many consider to
have been an incorrect direction in the first place.  Some build tools won't
work with star imports, and using them can make some optimizations more
difficult.  By avoiding this pattern here, hopefully we'll avoid moving back
in that direction inadvertently.

Here's a read I found at one point which roughly explains:

https://itnext.io/great-import-schism-typescript-confusion-around-imports-explained-d512fc6769c2

packages/apollo-server-core/src/types.ts

@@ -6,7 +6,9 @@ import {
   GraphQLParseOptions,
 } from 'graphql-tools';
 import { ConnectionContext } from 'subscriptions-transport-ws';
-import * as WebSocket from 'ws';
+// The types for `ws` use `export = WebSocket`, so we'll use the
+// matching `import =` to bring in its sole export.
+import WebSocket = require('ws');
 import { GraphQLExtension } from 'graphql-extensions';
 export { GraphQLExtension } from 'graphql-extensions';