javascript: add change note

Author: Eliav2

javascript/ql/lib/change-notes/released/2.6.15.md

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+## 2.6.15
+
+### Minor Analysis Improvements
+
+- JavaScript `DataFlow::globalVarRef` now recognizes `document.defaultView` as an alias of `window`, allowing flows such as `document.defaultView.history.pushState(...)` to be modeled and found by queries relying on `globalVarRef("history")`.