Distinguish instance- from direct-invokable types

[dfreeman]

Currently the two types of resolvable value are Invokable<T> and new () => Invokable<T>. This largely works well, with one main exception: it makes {{this}} look like an invokable value in component templates, since the instance type is itself Invokable<T>.

To fix this, we can distinguish between the normal case of invokable values that are wrapped in a constructor (which covers user-defined components, helpers and modifiers) and "direct invokable" values that are largely used internally to represent intrinsics like {{each}} and {{fn}}.

Plain function usage, as in GlimmerX, is unaffected by this change.