[performance-markers] initial proposal (WIP)

Idea brought up by @justinfagnani [here](https://bsky.app/profile/justinfagnani.com/post/3lixsva6pb22b). Leaving as draft while I review other proposals in this repo and the prior art (that I know of): - [User Timing API](https://developer.mozilla.org/en-US/docs/Web/API/Performance_API/User_timing) - [Performance API](https://developer.mozilla.org/en-US/docs/Web/API/Performance_API) - Chrome DevTools Performance Panel - [API Design Document](https://docs.google.com/document/d/1Fp4LLvq2VAv9ksgcxDGLf472Rbi9vz_Tqs74DdcT0hw) - [Final Documentation](https://developer.chrome.com/docs/devtools/performance/extension) - Lightning Web Components ([LWC](https://github.com/salesforce/lwc)) Implementation PRs - https://github.com/salesforce/lwc/pull/4535 - https://github.com/salesforce/lwc/pull/4541 - https://github.com/salesforce/lwc/pull/4544 - https://github.com/salesforce/lwc/pull/5143