ReadQuery: split out to fix record field warning

The use of `where_` in DbRequestBuilder issues a warning since
GHC 9.2 as per
https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0366-no-ambiguous-field-access.rst

To disambiguate that, move the type to a separate module and use
the record field names qualified. For symmetry, MutateQuery also
gets its own module.

src/PostgREST/Query/SqlFragment.hs

@@ -50,6 +50,7 @@ import PostgREST.DbStructure.Identifiers (FieldName,
                                           QualifiedIdentifier (..))
 import PostgREST.RangeQuery              (NonnegRange, allRange,
                                           rangeLimit, rangeOffset)
+import PostgREST.Request.ReadQuery       (SelectItem)
 import PostgREST.Request.Types           (Alias, Field, Filter (..),
                                           FtsOperator (..),
                                           JoinCondition (..),
@@ -61,7 +62,7 @@ import PostgREST.Request.Types           (Alias, Field, Filter (..),
                                           Operation (..),
                                           OrderDirection (..),
                                           OrderNulls (..),
-                                          OrderTerm (..), SelectItem,
+                                          OrderTerm (..),
                                           SimpleOperator (..),
                                           TrileanVal (..))