Allow configuring a base uri for MarkdownString

[mjbvz]

Feature request

Allow configuring a base uri that is used when rendering the contents of a MarkdownString. This uri would be used when resolving relative paths that appear as links or image links within the markdown string

Example use case

const md = new vscode.MarkdownString(`
[link](toSiblingFile.md)

![img](./image/from/dir.gif)
`);

md.baseUri = vscode.window.activeTextEditor.document.uri

When rendered, link and img should both resolve to files next to the currently active file

Related issues

• "Error: Unable to resolve non-existing file" on JSDoc file link #130968
• Support local paths in hover MarkdownString #86564

[mjbvz]

The alternative today is that extensions have to parse the markdown and transform the uris themselves. This is difficult to get correct