feat(assert-injector): add assertInjector

[nartc]

closes #16

This PR adds a utility function assertInjector that abstracts assertInInjectionContext and guarantees an Injector after the utility is invoked

function injectDummy(injector?: Injector) {
  injector = assertInjector(injectDummy, injector);
  // ^ injector is guaranteed to be an Injector
  runInInjectionContext(injector, () => {
    // always run in a particular Injector's context
  })
}

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[eneajaho]

LGTM

Nit: we need to add comments to these functions so the devs can se the usage example directly in the dts files