Allow for custom reports to be generated & queued

Allowing individual execution contexts to construct and send their own reports would allow this API to provide a single, consistent approach for the reporting of important client-side information such as unhandled errors, timing metrics, and usage information.

One API I have in mind for this is loosely based on the Performance API and goes something like:

class Reporter extends EventHandler {
  constructor(options?: ReporterOptions);
  static report<T extends string>(type: T, options?: ReportOptions): CustomReport;
  report(type: T, options?: ReportOptions): CustomReport;
}

interface ReporterOptions {
  readonly name?: string;
  readonly detail?: any;
}

interface ReportOptions {
  readonly detail: any;
  readonly timestamp: DomHighResTimeStamp;
}

class CustomReport extends Report {
  readonly body: CustomReportBody;
}

class CustomReportBody extends ReportBody {
  readonly reporter?: ReporterOptions;
  readonly detail?: any;
}

An execution context may then initialize a new report as:

Reporter.report('unhandled-error');
// or
const myReporter = new Reporter({name: 'toolbar'});
myReporter.report('item-clicked', {detail: {'item-name': 'Save'}});

All custom reports' types would be prefixed with 'custom-' so that they are clearly differentiated from browser-native events. Individual Reporter instances provide additional details about the context of a report.

The "body" of these reports would include a "detail" entry containing the "detail" value from the report as well as a "reporter" entry which contains values from the ReporterOptions of the Reporter instance that constructed the report, if any.