[scoped-custom-element-registry] Add benchmarks

[kevinpschaaf]

Task: The scoped-custom-element-registry, while building upon native Custom Elements where possible, adds some indirection, which is expected to add some measurable cost to element upgrade. It would be good to have a benchmark that quantifies this cost vs. native CE, so users can be aware of the additional cost of scoping.

[stale]

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