Warn when an async matchCallback is used

[jeffposnick]

R: @philipwalton

Some dev-only logging to help developers who are writing their own matchCallback functions, and mistakenly write them as async functions.

[jeffposnick]

FWIW, this isn't something that TypeScript could catch. Right now we have:

export interface RouteMatchCallback {
  (options: RouteMatchCallbackOptions): any;
}

Ideally that any would be "any, excluding Promise", but that's not possible until microsoft/TypeScript#29317 is available.