src: align PerformanceState class name with conventions

Class names are written in UpperCamelCase. Otherwise, this looks like
it’s a variable, not a class name.

PR-URL: https://github.com/nodejs/node/pull/32539


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>