feat: extension support (#78)

pkg/download/testdata/extensions/basic/index.js

@@ -0,0 +1,13 @@
+gopeed.events.onResolve(async function (ctx) {
+    ctx.res = {
+        name: "test",
+        files: Array(2).fill(true).map((_, i) => ({
+                name: `test-${i}.txt`,
+                size: 1024,
+                req: {
+                    url: ctx.req.url + "/" + i,
+                }
+            }),
+        ),
+    };
+});