Function measurePerformance

measurePerformance(
    name: string,
    startMark: string,
    endMark: string,
): number | null
Measure duration between two marks

Creates a performance measure between two named marks and returns the duration.
Parameters
- name: string
  Name for the measure
- startMark: string
  Start mark name
- endMark: string
  End mark name
Returns number | null
Duration in milliseconds or null if measurement failed
Example
```
markPerformance('fetch-start');
await fetchData();
markPerformance('fetch-end');
const duration = measurePerformance('data-fetch', 'fetch-start', 'fetch-end');
console.log('Fetch took:', duration, 'ms');
```
- Defined in performance/metrics.ts:240