Allow validation of extensionless links to non-markdown syntax files

[twhittock-disguise]

When working with "markdown.preferredMdPathExtensionStyle": "removeExtension" set, and a mix of MDX and MD files, it is not possible to validate and autocomplete links to MDX files from MD files using this language service.

The root cause, at least for validation, seems to be https://github.com/microsoft/vscode-markdown-languageservice/blob/7156aba6bdffef89b9210def0e0dda785569d1ab/src/workspace.ts#L227

This function requires all extensionless links in MD files to be handled by the language service. However, MDX files can't be handled by this service.

Perhaps a new setting ("markdown.knownMdExtensions": ["md", "mdx"]) could be added to allow link validation to find the file?