Change img hrefs in markdown from file to vscode-file

[RyanAfrish7]

In markdown renderer, hrefs of images in html are transformed from file: to vscode-file: protocol.

This PR fixes #136027

[ghost]

All CLA requirements met.

[mjbvz]

Besides the comment:

1. Let's see if we can remove the other call to _href so that all images are handled in a consistent way
2. Make sure to add some test cases for this new logic. Here's the test file:

   vscode/src/vs/base/test/browser/markdownRenderer.test.ts

   Line 23 in 1eaf52c

   suite('MarkdownRenderer', () => {

[mjbvz]

This seems to break markdown text such as: text <b>bold</b> as it gets rendered as:

<p>text <b></b>bold</p>

Instead, I think you should move this logic after the full md has been rendered but before sanitization:

vscode/src/vs/base/browser/markdownRenderer.ts

Line 245 in d209aa2

let renderedMarkdown = marked.parse(value, markedOptions);

[mjbvz]

Thanks! Should be in the next insiders build and is scheduled to go out with VS Code 1.63