docs(changeset): Add .scopedSQL(sql => ...) method to PgSelectStep,…

… PgSelectSingleStep, PgUnionAllStep, PgUnionAllSingleStep; can be used to allow embedding of other _typed_ steps directly into an SQL expression without needing to use `$pgSelect.placeholder(...)` wrapper. Only works with steps that have a `pgCodec: PgCodec` property (typically those coming from PgSelectSingleStep accesses) since otherwise we don't currently know how to cast the value to Postgres.

.changeset/dull-moles-exercise.md

@@ -0,0 +1,11 @@
+---
+"@dataplan/pg": patch
+---
+
+Add `.scopedSQL(sql => ...)` method to PgSelectStep, PgSelectSingleStep,
+PgUnionAllStep, PgUnionAllSingleStep; can be used to allow embedding of other
+_typed_ steps directly into an SQL expression without needing to use
+`$pgSelect.placeholder(...)` wrapper. Only works with steps that have a
+`pgCodec: PgCodec` property (typically those coming from PgSelectSingleStep
+accesses) since otherwise we don't currently know how to cast the value to
+Postgres.