[css-align] place-items doesn't allow 'left | right' keywords

[javifernandez]

We decided to remove 'left' and 'right' keywords from the syntax clause so that we could use it in both, Self Alignment and Default Alignment properties' syntax. Since them, we explicitly add those keywords for the main/inline axis properties (justify-self and justify-items).

However, this change made the place-items grammar wrong, or at least, incomplete:

https://drafts.csswg.org/css-align/#place-items-property

[ normal | stretch | <baseline-position> | <self-position> ] [ normal | stretch | <baseline-position> | <self-position> ]?

[tabatkins]

Yup, the justify-items grammar was editted incorrectly (@fantasai figured that since left/right were already part of the legacy term, she didn't need to add it to <self-position> explicitly), and then we just simplified 'place-items' to just directly ref the grammars of align/justify-items, same as the other place-* properties do. (The explicitly-written grammars were necessary when <overflow-position> was re-orderable, so there was potential ambiguity, but we set the order by WG resolution a while back.)