Instrument editor startup performance

[koke]

While testing, I noticed the editor took around 10 seconds to load in my Moto G5 Plus. Besides investigating what's causing the slow performance, it would be good to add some metrics to understand how widespread is the problem.

I'd propose we add a new property to editor_session_start named startup_time_ms that counts the number of milliseconds from the time the user tries to open the editor until it is available and responsive. This roughly translates as the time between the editor session being initialized and the start event being sent (since it'll wait until first render to know which blocks are unsupported).