There is performance issue on $.isEmpty

[tasuku-s]

In my usecase (Angular, many editor, long text in a block), copy, paste and click any line in block that has too long lines makes browser freezing.

I found that isEmpty method in editor.js/src/components/dom.ts has bad algorithm to traverse nodes.

When there are nodes like A, B, C, D, E, then this method traverse nodes as A, B, C, D, E, B, C, D, E, C, D, E, D, E.

[gohabereg]

Resolved by #1096