### Changes made
This implements named grid lines (line names in `grid-template-*`),
named grid areas (`grid-template-areas`), and the ability to target
those using `grid-{row,column}-{start,end}`. It also includes a bunch of
miscelaneous fixes for `repeat(auto-fill | auto-fit, ...)` syntax as
that interacts with the specification of line names.
The actual layout implementation is in Taffy. The bulk of this PR is
updating Servo to translate (CSS Grid-related) Stylo types into Taffy
types using a new iterator-based API which uses iterators and lazy
translation for efficiency (which is more important now that we're
dealing with string data, even though they're `Atom`s).
### Testing
This functionality has lots of WPT tests. It fixes some seemingly random
CSS Grid tests that use named lines/areas even though that's not what
they're testing.
### Screenshots
wikipedia.org
<img width="1624" height="1056" alt="Screenshot 2025-07-27 at 20 03 16"
src="https://github.com/user-attachments/assets/2c50b96f-ae36-4405-ac48-b771bfdcb515"
/>
bbc.co.uk:
<img width="1624" height="1056" alt="Screenshot 2025-07-27 at 20 32 57"
src="https://github.com/user-attachments/assets/ba84e211-65d2-4411-95fb-7b9b91bea31c"
/>
theguardian.com:
<img width="1624" height="1056" alt="Screenshot 2025-07-27 at 20 33 29"
src="https://github.com/user-attachments/assets/e85daaa6-5fb0-45d4-b9ec-b22b38b087ec"
/>
---------
Signed-off-by: Nico Burns <nico@nicoburns.com>
This applies some fixes for CSS Grid percentage sizing. These fixes are
mostly within Taffy, but there are some changes in Servo to allow it to
communicate whether an item is replaced to Taffy.
It also updates Taffy to v0.8.0. Taffy has switched to a tagged pointer
representation of length/size styles. Much of the diff here is updating
Servo's type conversion code to use the new representation.
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by
`[X]` when the step is complete, and replace `___` with appropriate
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] There are tests for these changes OR
---------
Signed-off-by: Nico Burns <nico@nicoburns.com>
