Added support for multiple handlers

[spoenemann]

In some cases it is necessary to register multiple handlers for the same message type in the VS Code extension (not in the webview). The change includes the following:

• Store an array of message handlers for each message type.
• Add a parameter to filter by sender, for example to register a handler only for a specific webview instance.
• Return a Disposable that can be used to remove a message handler when it's no longer needed.
• Webview registration returns information on the message participant (webview id) that can be used for filtering)
• Requests can be handled only if there is exactly one message handler for which the configured sender matches the incoming message.

[dhuebner]

Looks really good. A possibility to register handlers for a special sender makes the library more flexible.

LGTM!