[css-anchor-position-1] Add missing span-left/span-right values to definition w3c#12751

Author: fantasai

css-anchor-position-1/Overview.bs

@@ -710,7 +710,7 @@ by specifying the rows and columns the region occupies as follows:
 		but refer to the box's own [=writing mode=].
 
 	: <dfn>span-start</dfn>, <dfn>span-end</dfn>, <dfn>span-self-start</dfn>, <dfn>span-self-end</dfn>
-	: <dfn>span-top</dfn>, <dfn>span-bottom</dfn>
+	: <dfn>span-top</dfn>, <dfn>span-bottom</dfn>, <dfn>span-left</dfn>, <dfn>span-right</dfn>
 	: <dfn>span-y-start</dfn>, <dfn>span-y-end</dfn>, <dfn>span-y-self-start</dfn>, <dfn>span-y-self-end</dfn>
 	: <dfn>span-x-start</dfn>, <dfn>span-x-end</dfn>, <dfn>span-x-self-start</dfn>, <dfn>span-x-self-end</dfn>
 	: <dfn>span-block-start</dfn>, <dfn>span-block-end</dfn>, <dfn>span-self-block-start</dfn>, <dfn>span-self-block-end</dfn>