[SxS] Implement adding tab to new split

Adds a split property to TabModel which gets set when executing the
"Split with Active Tab" context menu command. Also moves tabs to be
adjacent to each other when creating a split view. This CL does not
include observers of the split property, or any changes to the content
view based on split state.

Bug: 392937095
Change-Id: Id2ee26fac81005c931a68de5b0c8dee1dc404e32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6208373
Reviewed-by: Eshwar Stalin <estalin@chromium.org>
Commit-Queue: Emily Shack <emshack@chromium.org>
Reviewed-by: David Pennington <dpenning@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1413174}