Add Web SDK package

[martinkuba]

We currently have Node SDK, which bundles together packages that are relevant to instrumenting Node applications. It also provides a simplified configuration that makes it easier for users to get started.

I propose that we introduce a Web SDK package that provides the same benefits for web applications - to make it easier for users to get started quickly with web instrumentation. It would include the following:

• simplified configuration for WebTracerProvider
• simplified configuration for EventLoggerProvider
• start/shutdown methods
• automatically run resource detectors
• default resource detectors relevant to web applications (e.g. browser detector)

In addition, I think the following could also be the Web SDK's responsibility, but I think more discussion is needed:

• automatically setup default exporters
  • In browsers, the HTTP JSON exporters make most sense
• processors for attaching session.id on spans and events
  • This could be a separate package, but initially could live in this package
• mechanism for configuring session management (e.g. persisting across page loads, expiration)