Layout: Implement CSS Grid using taffy (#32619)

* Add layout.grid.enabled pref

Signed-off-by: Nico Burns <nico@nicoburns.com>

* Add taffy dependency

Signed-off-by: Nico Burns <nico@nicoburns.com>

* Import taffy <-> stylo conversion code from taffy_stylo crate

Signed-off-by: Nico Burns <nico@nicoburns.com>

* Add `Grid` variant to DisplayInside

Signed-off-by: Nico Burns <nico@nicoburns.com>

* Implement CSS Grid using Taffy

Signed-off-by: Nico Burns <nico@nicoburns.com>

Import full stylo_taffy crate

Signed-off-by: Nico Burns <nico@nicoburns.com>

Squashed PR feedback changes

Deduplicate is_document_only_whitespace

Signed-off-by: Nico Burns <nico@nicoburns.com>

Import taffy::AvailableSpace

Signed-off-by: Nico Burns <nico@nicoburns.com>

Rename FlexContext to TaffyContainerContext

Signed-off-by: Nico Burns <nico@nicoburns.com>

Eliminate references to flexbox in taffy/layout module

Signed-off-by: Nico Burns <nico@nicoburns.com>

Use constructors for geom types

Signed-off-by: Nico Burns <nico@nicoburns.com>

Remove comment about abspos elements splitting contiguous text runs

Signed-off-by: Nico Burns <nico@nicoburns.com>

Remove reference to flexbox in taffy/construct

Signed-off-by: Nico Burns <nico@nicoburns.com>

Deduplicate construction of flexbox/grid containers

Signed-off-by: Nico Burns <nico@nicoburns.com>

Make anonymous text runs InFlow

Signed-off-by: Nico Burns <nico@nicoburns.com>

Remove commented code

Signed-off-by: Nico Burns <nico@nicoburns.com>

Update comments

Signed-off-by: Nico Burns <nico@nicoburns.com>

Inline/vendor the stylo/taffy interop code

Signed-off-by: Nico Burns <nico@nicoburns.com>

* Update test expectations

Signed-off-by: Nico Burns <nico@nicoburns.com>

* Fix nits from PR review

Signed-off-by: Nico Burns <nico@nicoburns.com>

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Signed-off-by: Nico Burns <nico@nicoburns.com>

resources/prefs.json

@@ -103,6 +103,7 @@
   "layout.columns.enabled": false,
   "layout.css.transition-behavior.enabled": true,
   "layout.flexbox.enabled": true,
+  "layout.grid.enabled": false,
   "layout.legacy_layout": false,
   "layout.threads": 3,
   "layout.writing-mode.enabled": false,