[core] Monitor build time in each PR

[oliviertassinari]

Summary

Similar to #78 that is about bundle size, I believe we should eventually monitor the build time cost of Pigment CSS.

Examples

No response

Motivation

See the conversation in https://gitlab.com/gitlab-org/frontend/rfcs/-/issues/107 on adopting Tailwind CSS. Build time is one of the concerns:

How much slower will the build be as comparing from today? (we already have a really slow build due to how we compile JS code in WebPack, would that add to the matter? evaluating the entire codebase is not free)

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[rstarkov]

The cost of hot starts of eg the Vite dev server is another potential concern, for example it comes up here: mui/material-ui#38137 (comment)