Convert selecting a named tuple into a free object

[scotttrinh]

Right now we do not allow selecting shapes on named tuples, so in order to support FTS, the simplest way forward is to convert any named tuples to a free object using a for expression.

There is one outstanding issue https://github.com/edgedb/edgedb/issues/6530 that requires an additional (hopefully temporary) hack for the output of fts::search to work properly: the compiler complains that the object computed in the resulting free object is not distinct, so we apply distinct to all of the named tuple's properties which should be a no-op.

There is some talk about making the output of fts::search into a free object in it's own right in the future, which is fine: everything will just work but now the query builder will be able to select shapes from named tuples.