LibJS: Mark classes and virtual functions final where possible

These cases were found with GCC's `-Wsuggest-final-{types,methods}`
warnings, which catch calls that could have been devirtualized had we
declared the functions `final` in the source.

To reproduce, Link Time Optimization needs to be enabled. The easiest
way to achieve this is to set the `CMAKE_INTERPROCEDURAL_OPTIMIZATION`
cache variable to `ON`. The `.incbin` directive in LibCompress' Brotli
decompressor might needs to be changed to an absolute path for this to
work.

This commit also removes a pair of unused virtual functions.

Userland/Libraries/LibJS/Heap/Cell.h

@@ -87,7 +87,6 @@ public:
         virtual ~Visitor() = default;
     };
 
-    virtual bool is_environment() const { return false; }
     virtual void visit_edges(Visitor&) { }
 
     // This will be called on unmarked objects by the garbage collector in a separate pass before destruction.

Userland/Libraries/LibJS/Heap/MarkedVector.h

@@ -33,7 +33,7 @@ public:
 };
 
 template<typename T, size_t inline_capacity>
-class MarkedVector
+class MarkedVector final
     : public MarkedVectorBase
     , public Vector<T, inline_capacity> {