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/Runtime/Environment.h

@@ -63,8 +63,6 @@ protected:
     virtual void visit_edges(Visitor&) override;
 
 private:
-    virtual bool is_environment() const final { return true; }
-
     bool m_permanently_screwed_by_eval { false };
 
     GCPtr<Environment> m_outer_environment;