This PR adds Path.addRSuperellipse to dart:ui. This is needed to implement a parity class to RoundedRectangleBorder as discussed here.

An alternative to #166045, this PR does not diverge at the dl_builder level, but simply implements pathing for both dart:ui and impeller::PathBuilder::addRSuperellipse with the same code. This avoids the DrawRSuperellipse op having varying size depending on drawing style, although it doesn't have the benefit that @/jonahwilliams hoped to calculate the path less frequently.

I think this is a better approach for its simpleness. If we want to calculate tessellation/path less frequently, we might want to do it in a grander scale for all shapes in a separate PR.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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