[csswg-drafts] Pull Request: [css-align-3] Add left|right to *-position grammar

sideshowbarker has just labeled a pull request from sideshowbarker for https://github.com/w3c/csswg-drafts as "css-align-3":

== [css-align-3] Add left|right to *-position grammar ==
This change adds the `left` and `right` keywords to the grammar terms
`<self-position>` and `<content-position>`.

That addition to the grammar brings it into sync with the existing spec prose,
which already includes `left` and `right` among the *positional alignment*
keywords collection, and states, “Two grammar terms are used to denote this
collection of values: `<self-position>` [and] `<content-position>`”.

See https://github.com/w3c/csswg-drafts/pull/2169