add performanceservertiming

[keithamus]

In the hopes of getting a discussion going around PerformanceServerTiming this implements the basic types as specified in the WD spec.

From the contributor guidelines:

• Does the new objects or fields show up in mdn/browser-compat-data? If not, it's likely too soon. (See browser-compat-data)
• Is the IDL source from WHATWG? (No, W3C)
  • Are the additions available in at least two of Firefox, Safari and Chromium?
• Is the IDL source from W3C?
  • What stage of the W3C process is the proposal for these changes: We aim for Proposed recommendation, but can accept Candidate recommendation for stable looking proposals. (Working Draft)
  • If it's at Working draft the additions available in all three of Firefox, Safari and Chromium (Not Safari yet, but Chrome, Edge & Firefox)
• Could any types added at the global scope have naming conflicts? (Doubtful unless someone is polyfilling these?)
• Are the new features going to be used by a lot of people? (We'd like to use this on GitHub 🤷)

From the browser compat table on mdn this API is available in Firefox 61, Chrome 65, Edge 79

[orta]

This looks good to me, thanks for the well written PR description - we should consider making a PR template with that?