refactor: move dumpQi to Config.hs and clarify quoteQi

The `dumpQi` function is only used in the `Config.hs` module, so
it is moved there. This also adds a doctest for `quoteQi` and
clarifies its usage.

Signed-off-by: Taimoor Zaeem <taimoorzaeem@gmail.com>

test/doc/Main.hs

@@ -23,4 +23,5 @@ main =
     , "src/PostgREST/Query/SqlFragment.hs"
     , "src/PostgREST/Response.hs"
     , "src/PostgREST/Response/Performance.hs"
+    , "src/PostgREST/SchemaCache/Identifiers.hs"
     ]