resolves #214 fix page counter

According to the specification the counter named page can be explicitly defined in the page context:

A counter named page is automatically created and incremented by 1 on every page of the document, unless the counter-increment property in the page context explicitly specifies a different increment for the page counter.

This merge request makes the page counter increment configurable by using a variable in the CSS counter-increment: page var(--pagedjs-page-counter-increment);. The default value is 1.

I've created a PageCounterIncrement to set the variable --pagedjs-page-counter-increment when the property counter-increment is defined in a page context for the counter named "page".

I've updated TargetCounters to increment the value accordingly. Please note that the value can be negative (i.e. counter-increment: page -1).

I've also fixed a potential bug (still need to create a test case) in counters.js where number was defined as:

counter.increments[selector] = {
  selector: selector,
  number: number || 1
};

The issue here is that number can be 0 and 0 is falsy... so if you declare counter-increment: foo 0 it will actually increment foo by 1 😬 Instead I'm using the following code:

const number = declaration.value.children.getSize() > 1 ? declaration.value.children.last().value : 1;

If the value is defined (i.e. declaration.value.children.getSize() > 1 is true) then use whatever the value is, otherwise, if the value is missing, use a default value of 1.

resolves #214 (closed)