Add support for retrieving array length in msl backend (#806)

* Add support for arrayLength to the wgsl frontend

* Fix clippy warning

* Add draft support for array length to the msl backend

* Finalize support for array length in msl

* Convert buffer size to array length in msl backend

* Fix clippy warning

* Fix misleading documentation

* Changes based on review of PR

* Use a fake binding for sizes buffer in msl backend if necessary

* Only generate the msl buffer size structure if globals are present that have unsized arrays

* Make sure to generate separators

* Replace uses_unsized_buffers with !self.runtime_sized_buffers.is_empty()

* Clear self.runtime_sized_buffers

* Run snapshot of shadow.msl

* Add Expression::ArrayLength support to the spirv backend

* Remove wgsl target from the access snapshot test

* Update access.msl snapshot

* Update stack size for msl backend

* Update stack size again for msl backend