Support multiple&shared states

[hellozyemlya]

Sometimes, multiple views need to share same state, or use common(but view-bound) state alongside with view specific state.

It will be great to have an ability to share VSCodeWebviewBridge instance between multiple views, as well as have an ability to specify multiple "bridges" for view.

This will allow:

• sync state between views
• have better compositions in complex extensions with multiple views

ps: many thanks for interesting project, that is very close to what I wanted in my extension, and a good source of inspiration