[kumo] Fix tabs indicator entrance animation

[minwookshin]

Fixes #714.

Use @starting-style for the indicator entrance and position it with left so it scales around the selected tab. Disable the transition when reduced motion is requested.

The four browser regressions failed before the fix and pass now; the same cases pass with reduced motion. Package build, typecheck, lint and 1,205 unit tests passed (208 skipped).

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[mattrothenberg]

I really appreciate the thoughtful work here. We’ve actually decided not to restore the entrance animation and will remove the unused styles in #841 instead. Closing in favor of that simpler approach.