Improve AtRule#nodes type

[tim-we]

Parsing something like

@layer a, b, c;

with postcss will result in an AtRule object without a nodes property. This however is not reflected by the types.

This PR changes the type of AtRule#nodes from ChildNodes[] to ChildNodes[] | undefined and updates the documentation.

I had to change the types of Container#nodes (and undo that change for Root#nodes, Rule#nodes and Document#nodes), I did not find a better approach. Other things I have tried are #1921 and changing this:

declare class AtRule_ extends Container<ChildNode | undefined>

which sadly also does not work because ChildNode | undefined does not extend Node.

Closes #1920.

[tim-we]

I'll have a look at those failing tests 😩

[tim-we]

Fixed tests.