Remove borders on codeblocks

[jjspace]

Part of #115

The default styling of the markdown preview applies the widgets.border color to codeblocks in the markdown preview.

This PR just removes that border to match the fact that GitHub does not add borders to these blocks

Testing

• Add a color customization to your settings

  "workbench.colorCustomizations": {
      "widget.border": "#000"
  }

• Create a markdown file with a codeblock containing anything
• Check that there is no border on this branch

[jjspace]

Just bumping this PR, @mjbvz can you take a quick look at this please? 🙏

[mjbvz]

Thanks!