This commit introduces a new type `EnvironmentProviders` which can be used
in contexts where Angular accepted `Provider`s destined for
`EnvironmentInjector`s. This includes contexts such as `@NgModule.providers`
and `Route.providers`.

The new type is useful for preventing such providers from accidentally
ending up in `@Component.providers`. It can be used as the return type of
provider functions (such as `provideRouter`) to enforce this safety.

Because `Provider` allows `any[]` nested arrays, the compile-time safety
provided by `EnvironmentProviders` is easily circumvented. However, the
runtime shape of `EnvironmentProviders` is not compatible with component
injectors and will result in a runtime error if it leaks through (NG0207).

A new function `makeEnvironmentProviders` is used to construct this new type
from an array of providers.

The existing `importProvidersFrom` operation previously returned a very
similar type `ImportedNgModuleProviders` which had the same goal. This
machinery is switched over to use the new `EnvironmentProviders` interface
instead (in fact, `ImportedNgModuleProviders` is now just an alias to
`EnvironmentProviders`).

PR Close #47669