Content (HTML attr)/IDL (JS property) attribute events not synced to textarea

[WebCoder49]

Events are currently only synced from the code-input element to its contained textarea if addEventListener() is called. If events are defined in HTML markup as content attributes (<code-input onblur="foo()">), or as JavaScript properties AKA IDL attributes (codeInputElement.onblur = foo), they don't get passed on.

Some events like selectionchange work because they get bubbled up by default, but others like blur do not.

After a fix is implemented adding this to the tests should be on the TODO list.

[WebCoder49]

Need to be careful with not duplicating listeners.