Update benchmarks

[jonathantneal]

This PR adds two kinds of benchmarks to the project.

• Vanilla CSS-in-JS methods
• React CSS-in-JS libraries

The first set of benchmarks test the performance of CSS-in-JS using various DOM and CSSOM methods.

The second set of benchmarks test the performance of React CSS-in-JS libraries performing common actions. The tested libraries are currently Stitches v0.0.2, Stitches v0.0.3-canary.4, Styled Components v5.2.0, and Emotion v10.0.27.

[codesandbox-ci]

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Latest deployment of this branch, based on commit 39c84b0:

Sandbox	Source
benchmark-stitches-vs-styled-components	Configuration

[peduarte]

I was chatting to @hadihallak, and maybe instead of pushing to Codesandbox CI, we can set up Vercel.

Reasons:

• Vercel will run the app in production mode. Codesanbox runs in dev mode
• We should be able to use npm alias (npm:) so we can have multiple versions of Stitches in the same app (needs validating)
• It will simplify the app structure, no need to have the styled-components folder

What do you think?