Ignore <!DOCTYPE> warning

Using the Ace HTML editor with a snippet of HTML instead of a full document will show a warning about missing <!DOCTYPE>.

This issue was closed, as it's an upstream issue that is not active.

https://github.com/ajaxorg/ace/issues/2428

Here's a js snipped that could help: https://groups.google.com/g/ace-discuss/c/qOVHhjhgpsU

var session = editor.getSession();
session.on("changeAnnotation", function() {
  var annotations = session.getAnnotations()||[], i = len = annotations.length;
  while (i--) {
    if(/doctype first\. Expected/.test(annotations[i].text)) {
      annotations.splice(i, 1);
    }
  }
  if(len>annotations.length) {
    session.setAnnotations(annotations);
  }
});