Use ZWSP instead of WBR

[tushuhei]

This changes the default separator from WBR to ZWSP.

We observed that some screen readers split their text focus by chunks because of the inserted WBR elements. We also considered adding the aria-hidden="true" attribute to the WBR elements as a workaround, but the behavior was still not ideal and stable. On the other hand, the ZWSP approach does not split the text focus and works better because it does not alter the DOM tree structure. Considering that, I suggest to use ZWSP as the default separator for accessibility improvement.

Users can still use WBR tags if it's preferred by specifying the separator element.

import {jaModel, HTMLProcessingParser} from 'budoux';
const parser = HTMLProcessingParser(jaModel, {separator: document.createElement('wbr')});

[kojiishi]

LGTM w/nit, thanks!

[tushuhei]

Thanks for reviewing!