perf(ext/headers): use regex.test instead of .exec (#20125)

This PR improves the performance of `Headers.get` by using `Regex.test`
instead of `.exec`. Also replaced the `Map` used for caching with an
object which is a bit faster

**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.get           124.71 ns/iter   8,018,687.3 (115.11 ns … 265.66 ns) 126.05 ns 136.12 ns 142.37 ns
```

**1.36.1**

```
cpu: 13th Gen Intel(R) Core(TM) i9-13900H
runtime: deno 1.36.0 (x86_64-unknown-linux-gnu)

benchmark              time (avg)        iter/s             (min … max)       p75       p99      p995
----------------------------------------------------------------------- -----------------------------
Headers.get           218.91 ns/iter   4,568,172.3 (165.37 ns … 264.44 ns) 241.62 ns 260.94 ns 262.67 ns
```

```js
const headers = new Headers({
  "Content-Type": "application/json",
  "Date": "Thu, 10 Aug 2023 07:45:10 GMT",
  "X-Deno": "Deno",
  "Powered-By": "Deno",
  "Content-Encoding": "gzip",
  "Set-Cookie": "__Secure-ID=123; Secure; Domain=example.com",
  "Content-Length": "150",
  "Vary": "Accept-Encoding, Accept, X-Requested-With",
});

Deno.bench("Headers.get", () => {
  headers.get("x-deno");
});
```

0 files changed, 0 insertions, 0 deletions