layout: Support start and end values for flexbox align-self (#33032)

These are similar to `flex-start` and `flex-end`, but in `wrap-reverse`
situations, they are the opposite.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>

tests/wpt/meta/MANIFEST.json

@@ -122594,6 +122594,19 @@
         ],
         {}
        ]
+      ],
+      "self-align-start-end-flex-001.html": [
+       "e8ce996f40dd985b8d4d064bdb1bca3e8dfba8a0",
+       [
+        null,
+        [
+         [
+          "/css/css-align/self-alignment/self-align-start-end-flex-001-ref.html",
+          "=="
+         ]
+        ],
+        {}
+       ]
       ]
      }
     },
@@ -393249,6 +393262,10 @@
       "self-align-safe-unsafe-grid-003-ref.html": [
        "0d0b17977d291bcc98874f34eb5364e819050dcc",
        []
+      ],
+      "self-align-start-end-flex-001-ref.html": [
+       "1b8c2558c316b20177e5b6c6615193b7e9e978cf",
+       []
       ]
      }
     },