[[feature-optionaltypes]] libfoundation: Finalize optional typecodes.
[[feature-propersetvalueref]] libfoundation: MCSetRef -> "livecode.lang.bitset".
[[feature-propersetvalueref]] libfoundation: Add MCProperSetTypeInfo.

Add a named alias of MCProperListTypeInfo and its optional variant.

Note that this is currently not hugely useful, because calling
MCValueGetTypeInfo() on an MCProperSetRef will return
kMCProperListTypeInfo.