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>

tests/wpt/meta/css/css-grid/subgrid/line-names-001.html.ini

@@ -1,2 +1,2 @@
 [line-names-001.html]
-  expected: CRASH
+  expected: FAIL

tests/wpt/meta/css/css-grid/subgrid/line-names-002.html.ini

@@ -1,2 +0,0 @@
-[line-names-002.html]
-  expected: CRASH

tests/wpt/meta/css/css-grid/subgrid/line-names-003.html.ini

@@ -1,2 +0,0 @@
-[line-names-003.html]
-  expected: CRASH

tests/wpt/meta/css/css-grid/subgrid/line-names-004.html.ini

@@ -1,2 +1,2 @@
 [line-names-004.html]
-  expected: CRASH
+  expected: FAIL

tests/wpt/meta/css/css-grid/subgrid/line-names-005.html.ini

@@ -1,2 +1,2 @@
 [line-names-005.html]
-  expected: CRASH
+  expected: FAIL

tests/wpt/meta/css/css-grid/subgrid/line-names-009.html.ini

@@ -1,2 +1,2 @@
 [line-names-009.html]
-  expected: CRASH
+  expected: FAIL

tests/wpt/meta/css/css-grid/subgrid/line-names-014.html.ini

@@ -1,2 +0,0 @@
-[line-names-014.html]
-  expected: FAIL

tests/wpt/meta/css/css-grid/subgrid/line-names-015.html.ini

@@ -1,2 +0,0 @@
-[line-names-015.html]
-  expected: FAIL

tests/wpt/meta/css/css-grid/subgrid/parent-repeat-auto-fit-001.html.ini

@@ -1,2 +1,2 @@
 [parent-repeat-auto-fit-001.html]
-  expected: CRASH
+  expected: FAIL

tests/wpt/meta/css/css-grid/subgrid/parent-repeat-auto-fit-002.html.ini

@@ -1,2 +1,2 @@
 [parent-repeat-auto-fit-002.html]
-  expected: CRASH
+  expected: FAIL