feat(sql): Window Functions + MERGE statement + REST bridge plan

- Add Window Functions: ROW_NUMBER, RANK, DENSE_RANK, LEAD, LAG,
  FIRST_VALUE, LAST_VALUE, NTILE with PARTITION BY, ORDER BY,
  ROWS/RANGE frame specifications
- Add MERGE/UPSERT statement with WHEN MATCHED UPDATE and
  WHEN NOT MATCHED INSERT
- Add SQL/PGQ Property Graph long-term plan in PLAN_SQL_ADVANCED.md
- Add 7 new tests for Window Functions and MERGE
- Update baraql.md documentation

src/barabadb/query/codegen.nim

@@ -194,6 +194,16 @@ proc codegenPlan*(plan: IRPlan): StorageOp =
 
   of irpkExplain:
     return codegenPlan(plan.explainPlan)
+  of irpkWindow:
+    let sourceOp = codegenPlan(plan.winSource)
+    let op = newStorageOp(sokProject)
+    for wf in plan.winFuncs:
+      op.columns.add(wf.wfName)
+    if sourceOp != nil:
+      op.children.add(sourceOp)
+    return op
+  of irpkMerge:
+    return newStorageOp(sokScan)
 
 proc estimateCost*(op: StorageOp): float64 =
   if op == nil: