This patch implements two more selector features:
- "div + p" matches the <p> sibling immediately after a <div>.
- "div ~ p" matches all <p> siblings after a <div>.
"div p" now generates a Selector with two components where the second
component is a type=TagName, value="p", relation=Descendant.
We still don't handle matching of these, but at least we parse them.
The parser now kinda recognizes immediate child selectors, !important,
and various other little things.
It's still extremely far from robust and/or correct. :^)
This was a workaround to be able to build on case-insensitive file
systems where it might get confused about <string.h> vs <String.h>.
Let's just not support building that way, so String.h can have an
objectively nicer name. :^)
