Markdown link diagnostics false positive on checkboxes

[lucaspar]

This is a special case of microsoft/vscode#150672 that was missed, and since that conversation is locked, I'm opening a new issue: besides - and *, markdown lists might also start with +.

Steps to Reproduce:

1. Type + [x] TODO checkbox syntax in Markdown
2. The validator reports a No link definition found: 'x' error.