Patch for plain syntax highlight

[leovinogradov]

There is a bug both in V1 and V2 where if you have a syntax code block with some highlighted text in it and select a different language, then select 'plain', some syntax highlights are not removed.

Try it out on quilljs.org by making a code block, putting in some text like
{
"hello": "world"
}
select 'javascript' as the language, then select 'plain'

This PR fixes this behavior by making sure there is no inserts in the diff between old delta and new delta when highlighting as plain