layout(grid): implement named grid lines and areas (#38306)

### 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>

Cargo.lock

@@ -3204,9 +3204,9 @@ dependencies = [
 
 [[package]]
 name = "grid"
-version = "0.17.0"
+version = "0.18.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "71b01d27060ad58be4663b9e4ac9e2d4806918e8876af8912afbddd1a91d5eaa"
+checksum = "12101ecc8225ea6d675bc70263074eab6169079621c2186fe0c66590b2df9681"
 
 [[package]]
 name = "gstreamer"
@@ -8345,9 +8345,9 @@ dependencies = [
 
 [[package]]
 name = "taffy"
-version = "0.8.3"
+version = "0.9.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7aaef0ac998e6527d6d0d5582f7e43953bb17221ac75bb8eb2fcc2db3396db1c"
+checksum = "a13e5d13f79d558b5d353a98072ca8ca0e99da429467804de959aa8c83c9a004"
 dependencies = [
  "arrayvec",
  "grid",