Support Content-Length and Last-Modified for webview local resources

[Vtec234]

Closes #125992

To test (steps from @mjbvz):

1. At the top of your workspace, create markdown
2. Also create an image called img.png
3. In markdown preview, add the content:

<script>
  fetch('img.png', {
    method: 'HEAD'
}).then(async (e) => {
    console.log(e.status);
    console.log(e.headers.get('content-type'));
    console.log(e.headers.get('content-length'));
    console.log(e.headers.get('last-modified'));
})
</script> 

4. Open the markdown preview and make sure scripts are enabled (there should be a warning if they are not)
5. Open the developer tools, check that the header contents have been printed and are correct
6. Same steps for a GET request

[ghost]

All CLA requirements met.

[Vtec234]

Note the stronger type on body: Uint8Array. AFAICT that's the only type which is ever passed through in a response and rely on it being so to call Uint8Array.byteLength.

[Vtec234]

typo?

[Vtec234]

typo?

[mjbvz]

Thanks! Will be in the next VS Code insiders build and is scheduled for 1.58

[Vtec234]

Thanks! Is the macOS test failure not a concern?