fix(helper/streaming): remove slow types (#3100)

Author: yusukebe

src/helper/streaming/sse.ts

@@ -58,7 +58,7 @@ const run = async (
   }
 }
 
-const contextStash = new WeakMap<ReadableStream, Context>()
+const contextStash: WeakMap<ReadableStream, Context> = new WeakMap<ReadableStream, Context>()
 export const streamSSE = (
   c: Context,
   cb: (stream: SSEStreamingApi) => Promise<void>,

src/helper/streaming/stream.ts

@@ -1,7 +1,7 @@
 import type { Context } from '../../context'
 import { StreamingApi } from '../../utils/stream'
 
-const contextStash = new WeakMap<ReadableStream, Context>()
+const contextStash: WeakMap<ReadableStream, Context> = new WeakMap<ReadableStream, Context>()
 export const stream = (
   c: Context,
   cb: (stream: StreamingApi) => Promise<void>,