Rewrote fileToUint8Array function to be also NodeJS/Deno compatible. (#2117)

## Motivation for the change, related issues
Performing multipart upload and posting of data currently works on the
web (client) side. Doing it on PHP WASM instance running on NodeJS or
other server runtimes (except Deno) causes this issue due to [FileReader
not supported on the
runtime](https://developer.mozilla.org/en-US/docs/Web/API/FileReader#browser_compatibility):

```
ReferenceError: FileReader is not defined
    at file:///c/Users/alonbuella/Documents/projects/php-wasm-vercel/node_modules/@php-wasm/universal/index.js:1717:15
    at new Promise (<anonymous>)
    at fileToUint8Array (file:///c/Users/alonbuella/Documents/projects/php-wasm-vercel/node_modules/@php-wasm/universal/index.js:1716:10)
    at encodeAsMultipart (file:///c/Users/alonbuella/Documents/projects/php-wasm-vercel/node_modules/@php-wasm/universal/index.js:1702:38)
    at PHPRequestHandler.ce (file:///c/Users/alonbuella/Documents/projects/php-wasm-vercel/node_modules/@php-wasm/universal/index.js:2070:48)
    at PHPRequestHandler.le (file:///c/Users/alonbuella/Documents/projects/php-wasm-vercel/node_modules/@php-wasm/universal/index.js:2056:33)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
```

## Implementation details
This PR introduces an alternative to using FileReader in order to
determine the `Uint8Array` value for a `File` instance. This is creating
a new instance of `Uint8Array` from the file's buffer instead.

`File`'s
[arrayBuffer](https://developer.mozilla.org/en-US/docs/Web/API/Blob/arrayBuffer#browser_compatibility)
instance method and
[Uint8Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array#browser_compatibility)
class are both supported on NodeJS and Deno, and as well as on most
recent browsers.

## Testing Instructions (or ideally a Blueprint)
- Tested locally via `npm run test`
- CI

Author: mbuella

packages/php-wasm/universal/src/lib/encode-as-multipart.ts

@@ -47,11 +47,9 @@ export async function encodeAsMultipart(
 }
 
 function fileToUint8Array(file: File): Promise<Uint8Array> {
-	return new Promise((resolve) => {
-		const reader = new FileReader();
-		reader.onload = () => {
-			resolve(new Uint8Array(reader.result as ArrayBuffer));
-		};
-		reader.readAsArrayBuffer(file);
-	});
+	/**
+	 * @mbuella: Use File.arrayBuffer() to get a Uint8Array from a file, avoiding FileReader
+	 * which is browser-specific. This method is supported in major browsers and NodeJS/Deno runtimes.
+	 */
+	return file.arrayBuffer().then((fileBuffer) => new Uint8Array(fileBuffer));
 }