Add skipPaths option for markdown link validation (#149859)

The new `markdown.experimental.validate.fileLinks.skipPaths` setting lets you specify a list of paths (as globs) that should not be validation

This is useful since markdown is used in a range of environments, and sometimes you may need to link to paths that don't exist on disk but will exist on deployment

A few other changes here:

- Adds a quick fix that adds paths to `skipPaths`
- Rename existing settings to use the `.enabled` prefix