chore: add pnpm bench:compare script

[Rich-Harris]

Super-crude but we can iterate on it. Allows us to easily compare branches

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• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
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• Ideally, include a test that fails without this PR but passes with it.

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

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Latest commit: bb38174

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