`makeEnvironmentProviders` constructs the wrapped `EnvironmentProviders`
type, which can only be used in environment injectors (not element
injectors). It makes sense that `makeEnvironmentProviders` should be able
to accept existing `EnvironmentProviders`-wrapped providers, since it will
be providing the same guarantee, but the current types do not allow this.
This commit fixes the typings to allow nesting `EnvironmentProviders` and
adds a test to verify that it will work.
PR Close#48720
Alex Rickabaugh
f00bf71411