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>

components/layout/flow/inline/construct.rs

@@ -672,7 +672,7 @@ where
 
 /// Given a string and whether the start of the string represents a word boundary, create a copy of
 /// the string with letters after word boundaries capitalized.
-fn capitalize_string(string: &str, allow_word_at_start: bool) -> String {
+pub(crate) fn capitalize_string(string: &str, allow_word_at_start: bool) -> String {
     let mut output_string = String::new();
     output_string.reserve(string.len());