fix(schedulers): no longer cause TypeScript build failures when Node …

…types aren't included

The `TimerHandle` type references a type from `@types/node` but that
package might not be available, using the return type of `setTimeout`
achieves the same result but doesn't require `@types/node` to be
installed.

src/internal/scheduler/timerHandle.ts

@@ -1 +1 @@
-export type TimerHandle = number | NodeJS.Timeout;
+export type TimerHandle = number | ReturnType<typeof setTimeout>;