Re: [css-values] Adding a <length-percentage> production

Actually, it turns out that the 'word-spacing' issue was fixed back in
2012 <https://github.com/w3c/csswg-drafts/commit/3090b58d855265a1d94e02cc5d2780fa0b9db16b>.
That was the only case I knew of that didn't automatically work for
calc().

That said, %s can still stand on their own, like in the last two
arguments of hsl(), and Houdini is still helped by having a single
nonterminal for this, so I've still added the productions to V&U.

~TJ