fix(js): expose CSSStyleDeclaration as a real global interface

CSSStyleDeclaration was added to the _preHideInternals non-enumerable
pre-declaration list (commit c7e7c70) on the assumption that its later
`globalThis.X = X` assignment would fill in the value. But it was the only
one of the 22 WebIDL interfaces with no such assignment, so its value stayed
undefined: `typeof window.CSSStyleDeclaration` was "undefined" while
`'CSSStyleDeclaration' in window` was true, and because it is the type of
element.style / getComputedStyle(), `el.style instanceof CSSStyleDeclaration`
threw "Right-hand side of 'instanceof' is not callable" (Chrome returns true).

Assign globalThis.CSSStyleDeclaration alongside the other interface globals.
The property was already declared writable/configurable, so the assignment
only updates the value and it remains enumerable:false, like a real browser.

Fixes #546

crates/obscura-js/js/bootstrap.js

@@ -6041,6 +6041,12 @@ globalThis.DocumentType = DocumentType;
 globalThis.Node = Node;
 globalThis.Element = Element;
 globalThis.Document = Document;
+// CSSStyleDeclaration is the type of element.style and getComputedStyle(); it is
+// pre-declared non-enumerable in _preHideInternals, but unlike the other WebIDL
+// interfaces it had no value assignment, leaving `window.CSSStyleDeclaration`
+// undefined (so `el.style instanceof CSSStyleDeclaration` threw). Assigning here
+// only fills the value; the property stays enumerable:false, matching Chrome.
+globalThis.CSSStyleDeclaration = CSSStyleDeclaration;
 globalThis.XPathResult = globalThis.XPathResult || class XPathResult {};
 Object.assign(globalThis.XPathResult, {
   ANY_TYPE: 0,