Removed JsonAny from message signatures

[spoenemann]

With the published version 0.2.0, I get an error with the following code:

export interface SprottyDiagramIdentifier {
    clientId: string,
    diagramType: string,
    uri: string
}

export const DiagramIdentifierNotification: NotificationType<SprottyDiagramIdentifier> = { method: 'SprottyDiagramIdentifier' };

Error:

Type 'SprottyDiagramIdentifier' does not satisfy the constraint 'JsonAny'.
  Type 'SprottyDiagramIdentifier' is not assignable to type 'JsonMap'.
    Index signature for type 'string' is missing in type 'SprottyDiagramIdentifier'. ts(2344)

This change should fix that. The motivation behind JsonAny was to enforce the objects to be JSON-serializeable, but obviously this not easily possible.

[dhuebner]

Looks good