layout: capitalize string for TextTransformCase::Capitalize in fn rendered_text_collection_steps (#37486)

Previously, `rendered_text_collection_steps` ignores
`TextTransformCase::Capitalize` due to limitation of iterator. Now we
handle the case outside.

Testing: Added a new test as not covered by existing wpt-test, except
for the indirectly related WebDriver test.
`./mach test-wpt -r
tests\wpt\tests\webdriver\tests\classic\get_element_text\get.py
--product servodriver`

Fixes: #37469

---------

Signed-off-by: Euclid Ye <yezhizhenjiakang@gmail.com>

tests/wpt/meta/MANIFEST.json

@@ -612692,6 +612692,13 @@
         ]
        ]
       },
+      "text-transform-capitalize-036.html": [
+       "a47dc35b949b56c734a0090c9c7a4932c745c4cf",
+       [
+        null,
+        {}
+       ]
+      ],
       "text-transform-upperlower-107.html": [
        "791edd14c0e144a945b4766a338725bca13da6bd",
        [