Support 'px' format in lineHeight option

Now the LineHeight can only be propagated to the number type like 1.2.

There is a very uncomfortable scenario:

I chose to use DOM to render，Then the contents of the subdiv of xterm-rows, as shown below, are determined based on the configuration items, such as 1.2, and the calculated value is 21px：

I want to set LineHeight to 23px, then I need to set LineHeight to 1.3, but I don't know how to turn 23 to 1.3