[RULE REQUEST] Flag HTML in markdown

[michaelcfanning]

The SARIF spec is quite clear that producers shouldn't emit HTML and that producers should be extremely wary of its inclusion.

Static Analysis Results Interchange Format (SARIF) Version 2.1.0 (oasis-open.org)

We should update our validator to raise an error on detecting any HTML constructs in markdown message properties.

@cfaucon

[KalleOlaviNiemitalo]

Don't flag escaped HTML though.

Use the \<kbd\> tag.

```HTML
<p>Press <kbd>Enter</kbd> when finished.</p>
```

[michaelcfanning]

Very good call-out.