Looking for some advice on how best to fix -- discussion after the form.

Describe the bug
If the Rouge Redcarpet plugin tries to guess the code language and the guess is ambiguous then it propagates an exception back up the stack which abandons the entire document.

To Reproduce

1. Set up Redcarpet with Rouge plugin.
2. Pass markdown through Redcarpet:

```
< !DOCTYPE math>
```

(without the space between < and !)

To see this particular ambiguity without redcarpet:

echo '<!DOCTYPE math>' | rougify highlight

Expected behavior
Fall back to either plaintext lexer or one of the ambiguees.

System
Any, though only seen in practice since Mason lexer added in Rouge 3.7.

The redcarpet plugin is very simple -- code is the stuff from within the triple backticks and language is the optional language identifier.

So this just calls Lexer.find_fancy. In the case when no language is supplied by the user we go to Lexer.guess who raises an exception if there is ambiguity.

The particular ambiguity with DOCTYPE has happened because both the xml and mason lexers respond to 'detect' for non-xml/html doctypes.

My first reactive fix is to delete Lexers::Mason.detect? -- I don't think it can be right for this dialect to claim all unclaimed doctypes? But I barely understand what Mason is.

Then a wider question: do users of find_fancy expect it to raise exceptions? It's not documented as such and none of the Rouge-internal users seem to be coded that way. So maybe we should change that interface to return one of the ambiguous lexers. This is a very old public interface though -- probably better to update the docs and update its callers to handle the exception and use one of the lexers therein?