Add Utility Function to Ensure Exact Number of Bytes Read from File

**Is your feature request related to a problem? Please describe.**

Currently, Deno provides `Deno.readFile()`, which reads an entire file into a buffer, and `Deno.FsFile.read()`, which reads a portion of a file into a buffer. However, `Deno.FsFile.read()` may return before reading the full requested number of bytes. This creates a challenge when attempting to reliably read *exactly* a specified number of bytes, requiring users to write custom logic to ensure they handle partial reads correctly.

**Describe the solution you'd like**

I propose adding a utility function to the standard library that abstracts away the complexity of handling partial reads, ensuring that the desired number of bytes is read into a buffer. This function might behave similarly to the following code:

```typescript
export async function read(file: Deno.FsFile, buffer: Uint8Array): Promise {
let offset = 0
while (offset < buffer.byteLength) {
const bytesRead = await file.read(buffer.subarray(offset))
if (bytesRead == null) break
offset += bytesRead
}
return buffer.subarray(0, offset)
}
```

This function would ensure that the buffer is filled (if possible) or gracefully handle end-of-file conditions without requiring developers to write custom loops.

**Describe alternatives you've considered**

Alternatives include manually implementing this logic in every project, as illustrated above. However, this approach leads to code duplication and potential bugs across codebases.