Support image resizing with width and height keyword

[yhatt]

This PR will support image resizing with width and height keyword as like as the other image keywords.

Originally in the pre-released Marp, it has supported to resize by percentage keyword relative to image size. It is realized by using zoom CSS property. However, it does not support in Firefox. Marpit cannot adopt the same way because of supporting cross-browser.

As the alternative way, we will support width & height keyword and w & h shorthands. You can specify these CSS declarations by the absolute unit.

![width:250px height:300px](image.png)
![w:250 h:300](image.png)

We had supported percentage value at first, but it doesn't support now. The percentage styling would not cause reflow with an incremental update in Firefox.

In regular background images, It also can use as the shortcut of backgroundSize spot directive. It also can use percentage value.

![bg w:50% h:75%](background.png)

In advanced backgrounds, the size of each image could specify separately.

ToDo

• Organize README to update section about images
  • The updated docs is going to feedback to https://marpit.marp.app/.