Add legacy widget to calendar transform

[jorgefilipecosta]

Based on: #15396
Should be merged after the other PR lands.

This PR makes use of the current transforms mechanism to allow transforming legacy widgets containing a calendar into a calendar block.

Testing

I pasted the following block in the code editor:

<!-- wp:legacy-widget {"identifier":"WP_Widget_Calendar","instance":{"title":"My calendar"},"isCallbackWidget":false,"hasEditForm":true} /-->

I was able to transform the legacy widget into a calendar block and also got a heading block with the title.

When I pasted this block and transformed it I only got a calendar:

<!-- wp:legacy-widget {"identifier":"WP_Widget_Calendar","instance":{"title":"My calendar"},"isCallbackWidget":false,"hasEditForm":true} /-->

When I pasted this blocks:

<!-- wp:legacy-widget {"identifier":"WP_Widget_Tag_Cloud","instance":{"title":"Tags","count":0,"taxonomy":"post_tag"},"isCallbackWidget":false,"hasEditForm":true} /-->

<!-- wp:legacy-widget {"identifier":"WP_Widget_Archives","instance":{"title":"My archives","count":0,"dropdown":0},"isCallbackWidget":false,"hasEditForm":true} /-->

I checked there was no option to convert them into a calendar.

[noisysocks]

This needs to be rebased now that #15396 is merged 🙂

[jorgefilipecosta]

Hi @noisysocks this PR was rebased and is ready for a review.