Fixes#8445
The only attributes I found that we have implemented that uses non-zero
dimenion attributes:
* `width` for `<td>` and `<th>` (table cells)
* `width` for `<table>`
I updated these implementations to use the new non-zero dimension
attribute parsing and added associated regression tests.
Updated string_cache, html5ever, xml5ever and selectors in Cargo.toml files and Cargo.lock.
Removed references to string_cache_plugin.
Import atom! and ns! from string_cache.
Replaced ns!("") by ns!().
Replaced ns!(XML) and co by ns!(xml) and co.
Replaced atom!(foo) by atom!("foo").
Replaced Atom::from_slice by Atom::from.
Replaced atom.as_slice() by &*atom.
