[css-lists] Design a 'counter' shorthand

[tabatkins]

In #8997 we resolved to add a 'counter' shorthand to let you set the counter-* styles together, since the grammar of counter-* properties makes them impossible to extend.

Here's my proposed syntax:

counter: <single-counter>#;
<single-counter> = <custom-ident> <integer>{0,2} <counter-style-name>?

The integers set the reset and the increment, in that order. If omitted they default to 0 and 1, same as the defaults in the longhands.

Questions:

• do we want to bake counter-set into this at all? It, fundamentally, has nothing to do with creating a counter, it's only useful on a different element from what's creating the counter. I have a slightly different grammar proposal if we want to make that possible. I propose, tho, that we don't do so, and have this shorthand only reset -reset, -increment, and -style.

Explicitly pinging @nt1m since they pushed the resolution.

[fantasai]

I think we can safely allow reordering the counter style and the integers.

[fantasai]

And yes, if you're incorporating counter-increment, incorporate counter-set also. Even if it's just as a reset-only property.