I had accidentally parsed it in `parse_builtin_or_dynamic_value()`
instead of `parse_length()` before, which was confusing, so now it's
parsed along with other Lengths.
Whether it should be a Length is up for debate, and has been tripping me
up a few times, but a lot of code expects it to be one. For now, an
'auto' Length value (or any other value which overloads `is_auto()`)
also claims to be a `ValueID::Auto` identifier.