Fix TypeScript compatibility issue with ITelemetryPlugin interface

[MSNev]

As part of some extension issues like

• Type 'AngularPlugin' is not assignable to type 'ITelemetryPlugin', Angular 15 ( "@angular/common": "15.1.0") error when trying to build the project applicationinsights-angularplugin-js#97

With the workaround PR

• Fix TypeScript compatibility issue with ITelemetryPlugin interface for Angular 15 applicationinsights-angularplugin-js#206

Update the BaseTelemetryPlugin to

• Make the setNextPlugin an Optional (implemented function)
• Mark the setNextPlugin as deprecated in the interface and all implementations (keeping the current usage)
• Fix typo in the interface stating that this the function "now optional" not "not optional"