Use sessionId instead of targetId to associate CDP client with a Context

[bwalderman]

The _handleDetachedFromTargetEvent function in browsingContextProcessor.ts uses the targetId parameter on the detachedFromTarget event which is depreacted. The sessionId parameter is recommended instead. This is likely because CDP supports multiple CDP sessions attached to the same Target (although we only use 1 CDP session for our purposes).

This tracks updating our code to use the sessionId parameter here instead. We would likely need to maintain a map in the browsingContextProcessor mapping CDP session IDs to Context objects.