perf(ext/headers): cache iterableHeaders for immutable Headers (#20132)

This PR caches `_iterableHeaders` for immutable `Headers` increasing the
performance of `fetch` & server if headers are iterated.

Should close #19466 

I only cached immutable headers to address this comment
https://github.com/denoland/deno/issues/19466#issuecomment-1589892373
since I didn't find any occurrence of header mutation on immutable
headers. We can discuss caching for non-immutable, but I think this is a
great first step.

## BENCHMARK

### Server
```js
const addr = Deno.args[0] ?? "127.0.0.1:4500";
const [hostname, port] = addr.split(":");
const { serve } = Deno;

serve({ hostname, port: Number(port), reusePort: true }, (req) => {
  const headers = [...req.headers]; // req.headers are immutable, cannot set/append/delete
  return new Response("ok");
});

```
Used `wrk` with 5 headers
```
wrk -d 10s --latency -H "X-Deno: true" -H "Accept: application/json" -H "X-Foo: bar" -H "User-Agent: wrk" -H "Accept-Encoding: gzip, br" http://127.0.0.1:4500
```


**This patch**
```
Running 10s test @ http://127.0.0.1:4500
  2 threads and 10 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    70.18us   22.89us 679.00us   81.37%
    Req/Sec    71.55k     9.69k   82.18k    89.60%
  Latency Distribution
     50%   59.00us
     75%   89.00us
     90%   98.00us
     99%  159.00us
  1437891 requests in 10.10s, 193.35MB read
Requests/sec: 142369.83
Transfer/sec:     19.14MB
```
**main**
```
Running 10s test @ http://127.0.0.1:4500
  2 threads and 10 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   112.78us   36.47us   2.09ms   77.99%
    Req/Sec    44.30k     1.65k   49.14k    74.26%
  Latency Distribution
     50%   99.00us
     75%  136.00us
     90%  162.00us
     99%  213.00us
  890588 requests in 10.10s, 118.91MB read
Requests/sec:  88176.37
Transfer/sec:     11.77MB
```
### fetch

```js
const res = await fetch('http://127.0.0.1:4500');
Deno.bench("Headers iterator", () => {
  const i = [...res.headers]; // res.headers are immutable, cannot set/append/delete
});
```

**this patch**
```
cpu: 13th Gen Intel(R) Core(TM) i9-13900H
runtime: deno 1.36.1 (x86_64-unknown-linux-gnu)

benchmark             time (avg)        iter/s             (min … max)       p75       p99      p995
---------------------------------------------------------------------- -----------------------------
Headers iterator      329.5 ns/iter   3,034,909.0 (318.55 ns … 364.34 ns)  331.1 ns 355.72 ns 364.34 ns
```
**main**
```
cpu: 13th Gen Intel(R) Core(TM) i9-13900H
runtime: deno 1.36.1 (x86_64-unknown-linux-gnu)

benchmark             time (avg)        iter/s             (min … max)       p75       p99      p995
---------------------------------------------------------------------- -----------------------------
Headers iterator       2.59 µs/iter     386,372.1     (2.56 µs … 2.68 µs)   2.59 µs   2.68 µs   2.68 µs
```

1 files changed, 13 insertions, 1 deletions

diff --git a/ext/fetch/20_headers.js b/ext/fetch/20_headers.js
index fabd39c0e..9e8f994fe 100644
--- a/ext/fetch/20_headers.js
+++ b/ext/fetch/20_headers.js
@@ -44,6 +44,7 @@ const {
 
 const _headerList = Symbol("header list");
 const _iterableHeaders = Symbol("iterable headers");
+const _iterableHeadersCache = Symbol("iterable headers cache");
 const _guard = Symbol("guard");
 
 /**
@@ -229,6 +230,13 @@ class Headers {
   get [_iterableHeaders]() {
     const list = this[_headerList];
 
+    if (
+      this[_guard] === "immutable" &&
+      this[_iterableHeadersCache] !== undefined
+    ) {
+      return this[_iterableHeadersCache];
+    }
+
     // The order of steps are not similar to the ones suggested by the
     // spec but produce the same result.
     const headers = {};
@@ -264,7 +272,7 @@ class Headers {
       ArrayPrototypePush(entries, cookies[i]);
     }
 
-    return ArrayPrototypeSort(
+    ArrayPrototypeSort(
       entries,
       (a, b) => {
         const akey = a[0];
@@ -274,6 +282,10 @@ class Headers {
         return 0;
       },
     );
+
+    this[_iterableHeadersCache] = entries;
+
+    return entries;
   }
 
   /** @param {HeadersInit} [init] */