Support for Node.js v18

## Describe the bug
Initialization of `ffmpeg.wasm` seems to fail on Node.js v18.

> **Note**
> * [Node.js v18 recently became the "Active LTS" version](https://nodejs.org/en/blog/release/v18.12.0/)
> * [Changing the End-of-Life Date for Node.js 16 to September 11th, 2023](https://nodejs.org/ar/blog/announcements/nodejs16-eol/)

## To Reproduce
1. Prepare a Node.js v18 runtime environment
```console
$ node -v
v18.12.0
```

2. Create a package and install `ffmpeg.wasm`
```console
$ npm init -y
$ npm i @ffmpeg/ffmpeg @ffmpeg/core
```

3. Prepare `index.mjs` (ESM) or `index.js` (CJS) as follows

ESM

```js
import { createFFmpeg } from '@ffmpeg/ffmpeg';
const ffmpeg = createFFmpeg();
await ffmpeg.load();
```

CJS

```js
const { createFFmpeg } = require('@ffmpeg/ffmpeg');
const ffmpeg = createFFmpeg();
(async () => {
await ffmpeg.load();
})();
```

4. Start execution

```console
$ node --experimental-wasm-threads index.(m)js
```

## Expected behavior
No errors should occur.

## Screenshots

ESM

```console
$ node --experimental-wasm-threads index.mjs
/path/to/pkg-dir/node_modules/@ffmpeg/core/dist/ffmpeg-core.js:12
...

TypeError: fetch failed
at Object.fetch (node:internal/deps/undici/undici:11118:11) {
cause: Error: unknown scheme
at makeNetworkError (node:internal/deps/undici/undici:9273:35)
at schemeFetch (node:internal/deps/undici/undici:10610:18)
at node:internal/deps/undici/undici:10483:26
at mainFetch (node:internal/deps/undici/undici:10500:11)
at fetching (node:internal/deps/undici/undici:10457:7)
at Agent.fetch2 (node:internal/deps/undici/undici:10337:20)
at Object.fetch (node:internal/deps/undici/undici:11116:28)
at fetch (node:internal/process/pre_execution:216:25)
at /path/to/pkg-dir/node_modules/@ffmpeg/core/dist/ffmpeg-core.js:215:32
at /path/to/pkg-dir/node_modules/@ffmpeg/core/dist/ffmpeg-core.js:215:254 {
[cause]: undefined
}
}

Node.js v18.12.0
```

CJS

```console
$ node --experimental-wasm-threads index.js
(node:12484) ExperimentalWarning: The Fetch API is an experimental feature. This feature could change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
/path/to/pkg-dir/node_modules/@ffmpeg/core/dist/ffmpeg-core.js:12
...

RuntimeError: abort(TypeError: fetch failed). Build with -s ASSERTIONS=1 for more info.
at process.u (/path/to/pkg-dir/node_modules/@ffmpeg/core/dist/ffmpeg-core.js:25:109)
at process.emit (node:events:513:28)
at emit (node:internal/process/promises:149:20)
at processPromiseRejections (node:internal/process/promises:283:27)
at process.processTicksAndRejections (node:internal/process/task_queues:96:32)

Node.js v18.12.0
```

## Desktop (please complete the following information):
- OS: Windows 10/11
- Node.js: v18.12.0

## Additional context
On Node.js v16, no error occurs with both ESM and CJS.

-----
@jeromewu Thank you for your great work.

Not related to the Node.js version mentioned above, but estimating from the Lavf version, the FFmpeg version is probably around v4.3, which seems a little old (the latest FFmpeg is already v5.1).

Do you have any plans to support these upgrades?