Add ApiStream generator to interface with providers

src/api/transform/stream.ts

@@ -0,0 +1,16 @@
+export type ApiStream = AsyncGenerator<ApiStreamChunk>
+export type ApiStreamChunk = ApiStreamTextChunk | ApiStreamUsageChunk
+
+export interface ApiStreamTextChunk {
+	type: "text"
+	text: string
+}
+
+export interface ApiStreamUsageChunk {
+	type: "usage"
+	inputTokens: number
+	outputTokens: number
+	cacheWriteTokens: number
+	cacheReadTokens: number
+	totalCost?: number // openrouter
+}