We've long claimed to support this, but then silently ignored string
values, until 4cb2063577 which would
not-so-silently crash instead. (Oops)
So, actually pass the string value along and use it in the list marker.
As part of this, rename our `list-style-type` enum to
`counter-style-name-keyword`. This is an awkward name, attempting to be
spec-based. (The spec says `<counter-style>`, which is either a
`<counter-style-name>` or a function, and the `<counter-style-name>` is
a `<custom-ident>` that also has a few predefined values. So this is the
best I could come up with.)
Unfortunately only one WPT test for this passes - the others fail
because we produce a different layout when text is in `::before` than
when it's in `::marker`, and similar issues.
At computed-value time, this is converted to whatever the parent's
computed value is. So it behaves a little like `inherit`, except that
an inherited start/end value uses the parent's start/end, which might
be different from the child's.
