Convert iterable into an internal alias for Traversable|array (#7309)

This does a compile time transformation of ``iterable`` into ``Traversable|array`` which simplifies some of the LSP vari

Convert iterable into an internal alias for Traversable|array (#7309)

This does a compile time transformation of ``iterable`` into ``Traversable|array`` which simplifies some of the LSP variance handling.

The arginfo generation script from stubs is updated to produce a union type when it encounters the type ``iterable``
Extension functions which do not regenerate the arginfo, or write them manually are still supported by mimicking the compile time transformation while registering the function.

Type Reflection is preserved for single ``iterable`` (and ``?iterable``) to produce a ReflectionNamedType with name ``iterable``, however usage of ``iterable`` in union types will be converted to ``array|Traversable``

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