perf(runtime/spawn): collect output using `op_read_all` (#16596)

**This patch**
```
benchmark      time (avg)             (min … max)       p75       p99      p995
------------------------------------------------- -----------------------------
echo deno   23.99 ms/iter   (22.51 ms … 33.61 ms)  23.97 ms  33.61 ms  33.61 ms
cat 16kb    24.27 ms/iter    (22.5 ms … 35.21 ms)   24.2 ms  35.21 ms  35.21 ms
cat 1mb     25.88 ms/iter   (25.04 ms … 30.28 ms)  26.12 ms  30.28 ms  30.28 ms
cat 15mb    38.41 ms/iter       (35.7 ms … 50 ms)  38.31 ms     50 ms     50 ms
```

**main**
```
benchmark      time (avg)             (min … max)       p75       p99      p995
------------------------------------------------- -----------------------------
echo deno   35.66 ms/iter   (34.53 ms … 41.84 ms)  35.79 ms  41.84 ms  41.84 ms
cat 16kb    35.99 ms/iter   (34.52 ms … 44.94 ms)  36.05 ms  44.94 ms  44.94 ms
cat 1mb     38.68 ms/iter   (36.67 ms … 50.44 ms)  37.95 ms  50.44 ms  50.44 ms
cat 15mb     48.4 ms/iter   (46.19 ms … 58.41 ms)  49.16 ms  58.41 ms  58.41 ms
```

1 files changed, 3 insertions, 17 deletions

diff --git a/runtime/js/40_spawn.js b/runtime/js/40_spawn.js
index e262a1325..8f44c8929 100644
--- a/runtime/js/40_spawn.js
+++ b/runtime/js/40_spawn.js
@@ -12,12 +12,12 @@
     ObjectEntries,
     String,
     TypeError,
-    Uint8Array,
     PromisePrototypeThen,
     SafePromiseAll,
     SymbolFor,
   } = window.__bootstrap.primordials;
   const {
+    readableStreamCollectIntoUint8Array,
     readableStreamForRidUnrefable,
     readableStreamForRidUnrefableRef,
     readableStreamForRidUnrefableUnref,
@@ -64,26 +64,12 @@
     };
   }
 
-  async function collectOutput(readableStream) {
+  function collectOutput(readableStream) {
     if (!(readableStream instanceof ReadableStream)) {
       return null;
     }
 
-    const bufs = [];
-    let size = 0;
-    for await (const chunk of readableStream) {
-      bufs.push(chunk);
-      size += chunk.byteLength;
-    }
-
-    const buffer = new Uint8Array(size);
-    let offset = 0;
-    for (const chunk of bufs) {
-      buffer.set(chunk, offset);
-      offset += chunk.byteLength;
-    }
-
-    return buffer;
+    return readableStreamCollectIntoUint8Array(readableStream);
   }
 
   class Child {