Refactor TimelineSeparator to shared-components #15354

Job	Run time
Build Element-Web	1m 17s
end-to-end-tests	1m 24s
Downstream Playwright tests [element-modules]	-1s
Run Tests [${{ matrix.project }}] ${{ matrix.runner }}/${{ needs.build.outputs.num-runners }}	-1s
	2m 39s

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